fix(ci): trigger fastlane on merged main PRs

Run the Linux amd64 fast-lane after a PR is merged to main, while leaving unmerged PRs as no-ops.

Co-authored-by: JustMe <mr.toing@outlook.com>

ci/release-linux.sh

@@ -12,8 +12,9 @@ if [[ -n "$tag" ]]; then
   }
   version="${tag#v}"
 else
-  [[ "${CI_PIPELINE_EVENT:-}" == push && "${CI_COMMIT_BRANCH:-}" == main ]] || {
-    echo "untagged image builds are restricted to pushes on main" >&2
+  [[ ("${CI_PIPELINE_EVENT:-}" == push && "${CI_COMMIT_BRANCH:-}" == main) || \
+     ("${CI_PIPELINE_EVENT:-}" == pull_request_closed && "${CI_COMMIT_BRANCH:-}" == main && "${FASTLANE_MERGED_PR:-}" == true) ]] || {
+    echo "untagged image builds are restricted to main pushes and merged PRs" >&2
     exit 2
   }
   version=main