Disable WebAssembly tests on Ubuntu 20.04: libc is too old for LLVM 19

Author: jmillikin

tests/scripts/run_tests.sh

@@ -68,7 +68,7 @@ fi
 # to run out of disk space.
 #
 # Mitigate this by expunging the workspace before trying to build Wasm targets.
-if [[ -z ${toolchain_name} ]]; then
+if [[ -z ${toolchain_name} && -z ${DISABLE_WASM_TESTS} ]]; then
   # Redefine `test_args` without `--linkopt=-Wl,-v`, which breaks `wasm-ld`.
   #
   # https://github.com/llvm/llvm-project/issues/112836

tests/scripts/ubuntu_20_04_test.sh

@@ -34,6 +34,10 @@ apt-get -qq -y install apt-utils curl libtinfo5 libxml2 pkg-config zlib1g-dev >/
 # The above command gives some verbose output that can not be silenced easily.
 # https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=288778
 
+# The WebAssembly tests use an LLVM version that is too new for the GNU libc
+# distributed in Ubuntu 20.04.
+export DISABLE_WASM_TESTS=1
+
 # Run tests
 cd /src
 tests/scripts/run_tests.sh