build-go.sh: OS_GIT_COMMIT => SOURCE_GIT_COMMIT

[sosiouxme]

This will be supplied in downstream builds to relay the commit from the source git repo.

Fixes: bz 1709365

- How to verify it
Check that the resulting OCP image built binary reports a commit from this github repo.

- Description for the changelog
build-go.sh: use SOURCE_GIT_COMMIT if available

[cgwalters]

I liked the commit message on the other one better, and we should have been using cherrypick right?

Also while we'e reworking this I probably would have preferred adding SOURCE_DATE_EPOCH handling in addition. But eh.

/approve
/lgtm

[openshift-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cgwalters, sosiouxme

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[openshift-bot]

/retest

Please review the full test history for this PR and help us cut down flakes.

[sosiouxme]

I can probably add the EPOCH envvar in doozer pretty easily and you can use it at will.

[wking]

What happened to OS_GIT_COMMIT, after #738 explicitly pivoted to it?

[sosiouxme]

@wking builds from origin didn't like it, see #aos-art