Fix rules_java release pipeline config (#2137)

This enables making releases at specific points even if the branch has
moved ahead.

Before this change, the release would always be made at the HEAD of
chosen branch, we should instead to do so at the specific commit that
the workflow is executed on.

pipelines/rules_java-release.yml

@@ -28,7 +28,7 @@ steps:
     command: | 
       echo "+++ Checking out Git branch"
       git fetch origin ${BUILDKITE_BRANCH}
-      git checkout ${BUILDKITE_BRANCH}
+      git checkout ${BUILDKITE_COMMIT}
 
       echo "+++ Getting the latest rules_java version"
       contents=$(cat MODULE.bazel)
@@ -53,4 +53,4 @@ steps:
       echo "+++ Creating a release on GitHub"
       cd bazel-bin/distro
       relnotes=$(cat relnotes.txt)
-      GITHUB_TOKEN=\${github_token} github-release "bazelbuild/rules_java" \${version} "\${BUILDKITE_BRANCH}" "\${relnotes}" rules_java-\${version}.tar.gz      
+      GITHUB_TOKEN=\${github_token} github-release "bazelbuild/rules_java" \${version} "\${BUILDKITE_COMMIT}" "\${relnotes}" rules_java-\${version}.tar.gz