break after deleting cache entry to avoid attempt to delete cache entry again

[boegel]

This fixes #1775.

Without the break, the enhanced test fails with:

======================================================================
ERROR: test_module_caches (__main__.ModulesTest)
Test module caches and invalidate_module_caches_for function.
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Traceback (most recent call last):
  File "test/framework/modules.py", line 583, in test_module_caches
    invalidate_module_caches_for(test_mods_path)
  File "/Users/kehoste/work/easybuild-framework/easybuild/tools/modules.py", line 1150, in invalidate_module_caches_for
    key, subcmd, path, cache[key])
KeyError: ('MODULEPATH=/var/folders/8s/_frgh9sj6m744mxt5w5lyztr0000gn/T/eb-Jvxp9Y/eb-fN6MJC/modules_symlink:/var/folders/8s/_frgh9sj6m744mxt5w5lyztr0000gn/T/eb-Jvxp9Y/eb-fN6MJC/nosuchfileordirectory:/Users/kehoste/work/easybuild-framework/test/framework/modules', 'modulecmd', '')

----------------------------------------------------------------------

[boegel]

fix confirmed by @brettbode in #1775, going in

Thanks for reporting the bug in detail @brettbode!