Set up publishing of SNAPSHOT builds to GitHub Packages

* Added workflow for publishing `SNAPSHOT` builds.
* Added workflow for removing `SNAPSHOT` builds once a pull request has been merged.

Author: carlspring

.github/workflows/build.yml

@@ -1,7 +1,8 @@
 name: Build and test
 on:
   # Build PRs and branches.
-  pull_request:
+  pull_request_target:
+    types: [ opened, synchronize, reopened ]
     paths-ignore:
       - .github/workflows/deploy-tagged.yml
   push:
@@ -51,7 +52,7 @@ jobs:
     name: Publish snapshot to GitHub Packages
     runs-on: ubuntu-latest
     needs: build
-    if: github.event_name == 'pull_request' || (github.event_name == 'push' && github.ref == 'refs/heads/master')
+    if: github.event_name == 'pull_request_target' || (github.event_name == 'push' && github.ref == 'refs/heads/master') && github.event.pull_request.head.repo.full_name == github.repository
     permissions:
       contents: read
       packages: write
@@ -71,7 +72,7 @@ jobs:
       - name: Determine snapshot version
         id: snapshot-version
         run: |
-          if [ "${{ github.event_name }}" == "pull_request" ]; then
+          if [ "${{ github.event_name }}" == "pull_request_target" ]; then
             SNAPSHOT_VERSION="${{ steps.get-version.outputs.base_version }}-PR-${{ github.event.pull_request.number }}-SNAPSHOT"
           else
             SNAPSHOT_VERSION="${{ steps.get-version.outputs.base_version }}-SNAPSHOT"