Runtimes: in macOS smoke test point to module cache from compiler build

[edymtt]

This will ensure we are not racing with other runs running on the same builders.

Addresses rdar://163122836

[edymtt]

@swift-ci please smoke test

[edymtt]

@swift-ci please smoke test Linux

[edymtt]

It looks like that did not help, the issue still happens in the specified module cache

```[2025-10-21T17:35:01.948Z] error: generate-pcm command failed with exit code 1 (use -v to see invocation)
[2025-10-21T17:35:01.948Z] <module-includes>:1:9: note: in file included from <module-includes>:1:
[2025-10-21T17:35:01.948Z] 1 | #import "_stdlib.h"
[2025-10-21T17:35:01.948Z]   |         `- note: in file included from <module-includes>:1:
[2025-10-21T17:35:01.948Z] 2 | #import "xlocale/_stdlib.h"
[2025-10-21T17:35:01.948Z] 3 | #import "_abort.h"
[2025-10-21T17:35:01.948Z] 
[2025-10-21T17:35:01.948Z] 1 error generated.
[2025-10-21T17:35:01.948Z] /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/_stdlib.h:66:2: error: module file '/Users/ec2-user/jenkins/workspace/swift-PR-macos-smoke-test/branch-main/build/buildbot_incremental/swift-macosx-x86_64/module-cache/_signal-5U2D9VP0AFEQCM2WHN2LCZN8F.pcm' is out of date and needs to be rebuilt: module file has a different size than expected```

[edymtt]

@swift-ci please smoke test

[edymtt]

@swift-ci please smoke test

[edymtt]

@swift-ci please smoke test Linux

[etcwilde]

The runtimes should all be pointing to the same cache path. They're building for the same target against the same sysroot, so the cached PCMs should all be consistent. This kind of smells like a bug somewhere in the PCM generation pipeline.

If we do want to allow folks to override the module-cache location, let's use the name Swift_MODULE_CACHE_DIR since it's controlling compiler behavior instead of toggling feature support in each of these libraries.