[Chore] Re-enable arm64 bottle builds for BigSur

Problem: in c636b39 we disabled bottle builds for BigSur on amd64,
because they were no longer building, but the issue has now been solved.

Solution: re-enable the pipeline step to build those bottles.

.buildkite/pipeline-for-tags.yml

@@ -139,24 +139,24 @@ steps:
      automatic:
        limit: 1
 
- #  - label: Build Big Sur arm64 bottles
- #    key: build-bottles-big-sur-arm64
- #    if: build.tag =~ /^v.*/
- #    agents:
- #      queue: "arm64-darwin"
- #    commands:
- #    - nix develop .#autorelease-macos -c ./scripts/build-all-bottles.sh "arm64_big_sur"
- #    artifact_paths:
- #      - '*.bottle.*'
- #    retry:
- #      automatic:
- #        limit: 1
+ - label: Build Big Sur arm64 bottles
+   key: build-bottles-big-sur-arm64
+   if: build.tag =~ /^v.*/
+   agents:
+     queue: "arm64-darwin"
+   commands:
+   - nix develop .#autorelease-macos -c ./scripts/build-all-bottles.sh "arm64_big_sur"
+   artifact_paths:
+     - '*.bottle.*'
+   retry:
+     automatic:
+       limit: 1
 
  # We use the tag that triggered the pipeline here. Normally, this isn't very resilient,
  # but in 'scripts/sync-bottle-hashes.sh' it's only used for informational purposes
  - label: Add Big Sur bottle hashes to formulae
    depends_on:
- #    - "build-bottles-big-sur-arm64"
+   - "build-bottles-big-sur-arm64"
    - "build-bottles-big-sur-x86_64"
    if: build.tag =~ /^v.*/
    soft_fail: true # No artifacts to download if all the bottles are already built