[LLDB] Fix write permission error in TestGlobalModuleCache.py (#76171)

TestGlobalModuleCache.py, a recently added test, tries to update a
source file in the build directory, but it assumes the file is writable.
In our distributed build and test system, this is not always true, so
the test often fails with a write permissions error.

This change fixes that by setting the permissions on the file to be
writable before attempting to write to it.