build: make commit hash for googlapis configurable

[jingyih]

Optionally allow developers to specify a commit hash to pull from googleapis.

The default behavior is still the same - pull from HEAD.

Specifying a commit hash could be useful when

• HEAD is broken
• There is an open PR to update googleapis in mockgcp but the PR hasn't merge yet. Specifying a previous commit hash could improve the stability and consistency of proto generation.

[google-oss-prow[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: justinsb

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[justinsb]

I love it!

[jingyih]

/retest