Prevent the publish bazel binary pipeline from stamping Bazel version in the binaries (#1491)

In https://github.com/bazelbuild/continuous-integration/pull/1195 we add
`--stamp --embed_label=<version>` by parsing the branch name to fix
https://github.com/bazelbuild/bazel/issues/13712. This is also causing
the https://buildkite.com/bazel-trusted/publish-bazel-binaries pipeline
to publish binaries with version stamp from commits in release branches.
This could be confusing, because commits from `release-6.0.0` will
always have version `6.0.0`. This PR tries to prevent that.