Revert "Trigger buildkite on external PRs" (#500)

.github/workflows/buildkite-external-prs.yaml

@@ -1,23 +0,0 @@
-name: Trigger Buildkite on External PR
-
-on:
-  pull_request:
-    types:
-      - labeled
-
-jobs:
-  trigger:
-    # Only intra-repo PRs are allowed to have Buildkite run on them automatically
-    # In order to allow PRs from third parties to build on Buildkite, we require an org member add a label after the PR has been vetted.
-    # This will only happen when the label is added, so as to reduce the risk of an innocuous first commit with malicious follow-up commits (after the label was added).
-    # This means, if the contributor's PR needs changes, someone with the ability to change labels will need to _remove_ the `trigger buildkite` label, _and then re-add it_.
-    if: github.event.action == 'labeled' && github.event.label.name == 'trigger buildkite'
-    runs-on: ubuntu-latest
-    steps:
-      - name: Trigger a Buildkite Build
-        uses: "buildkite/trigger-pipeline-action@v1.5.0"
-        env:
-          BUILDKITE_API_ACCESS_TOKEN: ${{ secrets.TRIGGER_BK_BUILD_TOKEN }}
-          PIPELINE: "determinate-systems-inc/nix-installer"
-          COMMIT: ${{ github.event.pull_request.head.sha }}
-          MESSAGE:  ":github: Triggered from a GitHub Action"