Go build rules: only stamp actual go_binary outputs
Go build rules: only stamp actual go_binary outputs

Summary:
The build rules currently stamp all linked Go binaries with the data
from stable-status.txt and volatile-status.txt, but this isn't really
needed for test binaries.  Also, since all actions depending on
volatile-status.txt will have to rerun for each build since it embeds a
timestamp, this change will improve the runtime for a null Bazel build.

Reviewers: fromberger, shahms

Reviewed By: shahms

Differential Revision: https://phabricator-dot-kythe-repo.appspot.com/D865

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