Add explicit shebang for bash in go_indexer_test.bzl
Add explicit shebang for bash in go_indexer_test.bzl

Summary:
Bazel's sandbox mounts the users /bin and /bin/sh may not be bash
(on Ubuntu it is now dash: https://wiki.ubuntu.com/DashAsBinSh )

Reviewers: fromberger

Reviewed By: fromberger

Subscribers: fromberger, #core_team

Differential Revision: https://phabricator-dot-kythe-repo.appspot.com/D1362

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