[libc++] Deflake the Clang Modules CI job

[ldionne]

Summary

This re-introduces the workaround that had been introduced in d7ca140c0122
and then removed in 0c0628c92c0d, since it seems like it is needed after all.

[iana]

params.py looks fine to me. I don't understand the pipeline changes, but if those are on purpose then I trust they're necessary.

[ldionne]

In D157319#4566873, @iana wrote:

params.py looks fine to me. I don't understand the pipeline changes, but if those are on purpose then I trust they're necessary.

I'll revert the pipeline changes, the goal is only to avoid running the full pipeline on this change several times.

[ldionne]

I'm going to land this (without the pipeline changes) and monitor the libc++ CI pipeline to see whether that helps with flaky modular builds.

libcxx/utils/libcxx/test/params.py

Parameter(
         type=str,
         help="Whether to build the test suite with modules enabled. Select "
         "`clang` for Clang modules and `std` for C++23 std module",
         default=lambda cfg: next(s for s in _allModules if getModuleFlag(cfg, s)),
         actions=lambda enable_modules: [
             AddFeature("modules-build"),
             AddCompileFlag("-fmodules"),
             AddCompileFlag("-fcxx-modules"), # AppleClang disregards -fmodules entirely when compiling C++. This enables modules for C++.
+            # Note: We use a custom modules cache path to make sure that we don't reuse
+            #       the default one, which can be shared across CI builds with different
+            #       configurations.
+            AddCompileFlag(lambda cfg: f"-fmodules-cache-path={cfg.test_exec_root}/ModuleCache"),
         ]
         if enable_modules == "clang"
         else [
             AddFeature("use_module_std"),
             AddCompileFlag("-DTEST_USE_MODULE"),
             AddCompileFlag("-DTEST_USE_MODULE_STD"),
             AddCompileFlag(
                 lambda cfg: "-fprebuilt-module-path="