github-actions: fetch upstream release tags if running on fork

This ensures the cyrus build can correctly identify its version
from the release tags, without requiring the fork owner to keep
their fork up to date with release tags, which in turn prevents
test failures from cyrus reporting its own version incorrectly,
and Cassandane miscompensating.

.github/workflows/main.yml

@@ -31,6 +31,15 @@ jobs:
     - name: setup git safe directory
       shell: bash
       run: git config --global --add safe.directory /__w/cyrus-imapd/cyrus-imapd
+    - name: fetch upstream release tags
+      if: ${{ github.repository != 'cyrusimap/cyrus-imapd' }}
+      shell: bash
+      run: |
+        git remote add upstream https://github.com/cyrusimap/cyrus-imapd.git
+        # n.b. --no-tags does not mean "no tags", it means "no automatic tag
+        # following".  we're explicitly fetching the tags we want, we do not
+        # need every other tag that's reachable from them
+        git fetch --no-tags upstream 'refs/tags/cyrus-imapd-*:refs/tags/cyrus-imapd-*'
     - name: configure and build
       shell: bash
       run: |