Remove clang-18 from compilers CI

clang-18 has a bug that causes ruby_current_ec to sometimes be null when
using Ractors and crashes like this:

    <internal:ractor>:700: [BUG] Segmentation fault at 0x0000000000000030
    ruby 4.0.0dev (2025-11-21T06:49:14Z master bcc7b20) +PRISM [x86_64-linux]

    -- Control frame information -----------------------------------------------
    c:0004 p:0003 s:0015 e:000014 l:y b:0001 METHOD <internal:ractor>:700
      me:
        called_id: receive, type: iseq
        owner class: 0x00007ff462dda500 T_CLASS/Ractor::Port
      self: 0x00007ff46146d068 ractor/port/Ractor::Port ractor/port
    c:0003 p:0008 s:0011 e:000010 l:y b:0001 METHOD <internal:ractor>:311
      me:
        called_id: receive, type: iseq
        owner class: 0x00007ff462ddae60 T_CLASS/(anon)
      self: 0x00007ff462ddaf00 T_CLASS/Ractor
    c:0002 p:0010 s:0007 e:000006 l:n b:---- BLOCK  bootstraptest.test_ractor.rb_2354_1323.rb:9 [FINISH]
      self: 0x00007ff46146d090 ractor/Ractor r:68
      lvars:
        j: T_FIXNUM 66
    c:0001 p:---- s:0003 e:000002 l:y b:---- DUMMY  [FINISH]
      self: T_NIL

    -- Ruby level backtrace information ----------------------------------------
    bootstraptest.test_ractor.rb_2354_1323.rb:9:in 'block (2 levels) in <main>'
    <internal:ractor>:311:in 'receive'
    <internal:ractor>:700:in 'receive'

    -- Threading information ---------------------------------------------------
    Total ractor count: 7
    Ruby thread count for this ractor: 1

    -- Machine register context ------------------------------------------------
    RIP: 0x00007ff47c7df5f0 RBP: 0x000055d77ea5b4f0 RSP: 0x00007ff445fa3af0
    RAX: 0x0000000000000000 RBX: 0x000055d77e9fd068 RCX: 0x000055d77e9fd040
    RDX: 0x000055d77eb2ac40 RDI: 0x00007ff47cbe7700 RSI: 0x0000000000000000
      R8: 0x0000000000000000  R9: 0x0000000000000000 R10: 0x000055d77e9fc830
    R11: 0x93ba1054e59bfb14 R12: 0x000055d77ea5b4f0 R13: 0x00007ff445f82f20
    R14: 0x00007ff4614cf668 R15: 0x000055d77e9fd040 EFL: 0x0000000000010246

    -- C level backtrace information -------------------------------------------
    libruby.so.4.0(rb_print_backtrace+0x14) [0x7ff47c8cbd18] vm_dump.c:1105
    libruby.so.4.0(rb_vm_bugreport) vm_dump.c:1450
    libruby.so.4.0(rb_bug_for_fatal_signal+0x162) [0x7ff47c70ce02] error.c:1131
    libruby.so.4.0(sigsegv+0x4a) [0x7ff47c82f20a] signal.c:948
    /lib/x86_64-linux-gnu/libc.so.6(0x7ff47c34a330) [0x7ff47c34a330]
    libruby.so.4.0(rb_ec_thread_ptr+0x0) [0x7ff47c7df5f0] vm_core.h:2092
    libruby.so.4.0(rb_ec_ractor_ptr) vm_core.h:2041
    libruby.so.4.0(rb_current_execution_context) vm_core.h:2110
    libruby.so.4.0(rb_current_ractor_raw) vm_core.h:2109
    libruby.so.4.0(rb_current_ractor) vm_core.h:2117
    libruby.so.4.0(ractor_unlock) ractor.c:110
    libruby.so.4.0(ractor_unlock_self) ractor.c:125
    libruby.so.4.0(ractor_wait) ractor_sync.c:1054
    libruby.so.4.0(ractor_wait_receive) ractor_sync.c:1113
    libruby.so.4.0(ractor_receive+0x25) [0x7ff47c7ded08] ractor_sync.c:1166
    libruby.so.4.0(ractor_port_receive) ractor_sync.c:143
    libruby.so.4.0(builtin_inline_class_700) ractor.rb:701
    libruby.so.4.0(invoke_bf+0x4) [0x7ff47c8a2060] vm_insnhelper.c:7534
    libruby.so.4.0(vm_invoke_builtin_delegate) vm_insnhelper.c:0
    libruby.so.4.0(vm_exec_core) insns.def:1674
    libruby.so.4.0(vm_exec_loop+0x0) [0x7ff47c89b868] vm.c:2784
    libruby.so.4.0(rb_vm_exec) vm.c:2787
    libruby.so.4.0(vm_invoke_proc+0x344) [0x7ff47c8b03f4] vm.c:1814
    libruby.so.4.0(thread_do_start_proc+0x17a) [0x7ff47c870bba] thread.c:593
    libruby.so.4.0(thread_do_start+0x162) [0x7ff47c87042f] thread.c:635
    libruby.so.4.0(thread_start_func_2) thread.c:686
    libruby.so.4.0(rb_native_mutex_lock+0x0) [0x7ff47c870fd1] thread_pthread.c:2238
    libruby.so.4.0(thread_sched_lock_) thread_pthread.c:403
    libruby.so.4.0(call_thread_start_func_2) thread_pthread_mn.c:466
    libruby.so.4.0(co_start) thread_pthread_mn.c:464

Author: peterzhu2118

.github/workflows/compilers.yml

@@ -109,7 +109,8 @@ jobs:
       - { uses: './.github/actions/compilers', name: 'clang 21', with: { tag: 'clang-21' } }
       - { uses: './.github/actions/compilers', name: 'clang 20', with: { tag: 'clang-20' } }
       - { uses: './.github/actions/compilers', name: 'clang 19', with: { tag: 'clang-19' } }
-      - { uses: './.github/actions/compilers', name: 'clang 18', with: { tag: 'clang-18' } }
+      # clang-18 has a bug causing ruby_current_ec to sometimes be null
+      # - { uses: './.github/actions/compilers', name: 'clang 18', with: { tag: 'clang-18' } }
       - { uses: './.github/actions/compilers', name: 'clang 17', with: { tag: 'clang-17' } }
       - { uses: './.github/actions/compilers', name: 'clang 16', with: { tag: 'clang-16' } }
       - { uses: './.github/actions/compilers', name: 'clang 15', with: { tag: 'clang-15' } }