A few `x_defs` attributes were missing on binaries, which is now fixed.
Also moved to stamping from `go_library` rules to `go_binaries` to ease
caching.
## Test plan
<!-- All pull requests REQUIRE a test plan:
https://docs.sourcegraph.com/dev/background-information/testing_principles
-->
CI + main-dry-run + locally tested + `strings github-proxy | grep
version` on `us.gcr.io/sourcegraph-dev/github-proxy:323c450504f8`