[build-script-helper] Use separate module cache

This will ensure that multiple build jobs running in parallel on the
same machine (like it could be the case in CI) do not interfere with
each other.

Addresses rdar://71373849

Author: edymtt

Utilities/build-script-helper.py

@@ -382,6 +382,9 @@ def build_using_cmake(args, toolchain_bin, build_dir):
   else:
     swift_flags.append('-O')
 
+  # Ensure we are not sharing the module cache with concurrent builds in CI
+  swift_flags.append('-module-cache-path "{}"'.format(os.path.join(build_dir, 'module-cache')))
+
   for target in targets:
     swift_flags.append('-target %s' % target)
     base_cmake_flags = ['-DCMAKE_Swift_FLAGS=' + ' '.join(swift_flags)]