Set SKIP_GIT_CLEAN to a string, for proper marshalling
Set SKIP_GIT_CLEAN to a string, for proper marshalling

Summary:
Failure to do so results in:

    error getting config: json: cannot unmarshal number into Go value
    of type string

Test Plan: Untested.

Reviewers: kylec

Reviewed By: kylec

Subscribers: changesbot

Differential Revision: https://tails.corp.dropbox.com/D158897

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