ci: Temporarily pin toolchain for mips r6

Author: taiki-e

.github/workflows/test-matrix.json

@@ -323,19 +323,19 @@
       "target": "mips64el-unknown-linux-gnuabi64"
     },
     {
-      "rust": "nightly",
+      "rust": "nightly-2026-01-28",
       "target": "mipsisa32r6-unknown-linux-gnu"
     },
     {
-      "rust": "nightly",
+      "rust": "nightly-2026-01-28",
       "target": "mipsisa32r6el-unknown-linux-gnu"
     },
     {
-      "rust": "nightly",
+      "rust": "nightly-2026-01-28",
       "target": "mipsisa64r6-unknown-linux-gnuabi64"
     },
     {
-      "rust": "nightly",
+      "rust": "nightly-2026-01-28",
       "target": "mipsisa64r6el-unknown-linux-gnuabi64"
     },
     {

tools/build.sh

@@ -325,6 +325,8 @@ build() {
         hexagon*) return 0 ;;
         # compiler SIGSEGV (last checked: nightly-2025-09-11)
         m68k-unknown-none-elf) return 0 ;;
+        # TODO(mips): compiler SIGILL with LLVM 22
+        mipsisa*) return 0 ;;
       esac
       args+=(-Z build-std="core")
     elif is_no_std "${target}"; then

tools/matrix.sh

@@ -342,6 +342,11 @@ for target in "${targets[@]}"; do
           nightly) toolchain=nightly-2024-08-30 ;;
         esac
         ;;
+      mipsisa*)
+        case "${toolchain}" in
+          nightly) toolchain=nightly-2026-01-28 ;; # TODO(mips): compiler SIGILL with LLVM 22
+        esac
+        ;;
     esac
     add_matrix
   done