Add an environment variable to point to alternate global configs

Summary: Add a way to specify a config location at runtime: an environment variable called `WATCHMAN_CONFIG_FILE`. This is useful both to isolate the test suite and to write tests that need certain global options to be set.

Test Plan: `arc unit --everything`, though the real test for it will come in an uncoming diff.

Reviewers: wez

Reviewed By: wez

Differential Revision: https://phabricator.fb.com/D900221

Task ID: 2647765

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extra : rebase_source : f7aa8474becefd97c0f6522ae3b88ab32dbd52e6
extra : amend_source : c8a287f7327fd1d4e78ed7f8b0349d5eb0d24db6