run_tests: leverage cipd when available for help2man

This tool isn't installed on CI bot images so we've been skipping it,
but this is causing people to not run tests locally, and ignore errors.
Use cipd to pull the tool in when available.

Then revert a recent man change that the tool rejects.

Change-Id: I1030d0070fd5a624656eba7434ae6ec99b2e3f2d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/582401
Reviewed-by: Greg Edelston <gredelston@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

.gitignore

@@ -6,6 +6,7 @@ __pycache__
 /dist
 .repopickle_*
 /repoc
+/.cipd_bin
 /.tox
 /.venv