CI: Disable CLANG64 MSYS2 environment until bug is resolved.

lld 17.0.1 searches for libraries to link first in the toolchain
directories before the local directory when building. The is a problem
for us because liblzma.a is installed in MSYS2 CLANG64 by default and
xz.exe will thus use the installed library instead of the one being
built.

This causes tests to fail when they are expecting features to be
disabled. More importantly, it will compile xz.exe with an incorrect
liblzma and could cause unexpected behavior by being unable to update
liblzma code in static builds. The CLANG64 environment can be tested
again once this is fixed.

Link to bug: https://github.com/llvm/llvm-project/issues/67779.

.github/workflows/windows-ci.yml

@@ -18,9 +18,11 @@ jobs:
       matrix:
         # Test different environments since the code may change between
         # them and we want to ensure that we support all potential users.
-        # clang64 currently fails due to a warning, so support will be
-        # added later.
-        msys2_env: [mingw64, mingw32, ucrt64, clang64, msys]
+        # clang64 builds are currently broken when building static libraries
+        # due to a bug in ldd search path:
+        # https://github.com/llvm/llvm-project/issues/67779
+        # TODO - re-enable clang64 when this is resolved.
+        msys2_env: [mingw64, mingw32, ucrt64, msys]
         build_system: [autotools, cmake]
 
     # Set the shell to be msys2 as a default to avoid setting it for