ci: run ci-release-build.sh on GitHub

This adds a GitHub action in order to run ci-release-build.sh on
release PRs (detected via the `kind/release` label).

ci/ci-release-build.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 set -euo pipefail
 
 # Makes sure that is_release_build is only set to yes in a release commit. A
@@ -10,6 +10,7 @@ set -euo pipefail
 HEAD=${PAPR_COMMIT:-HEAD}
 
 git log --format=%B -n 1 $HEAD > log.txt
+trap "rm -f log.txt" EXIT
 
 if grep -q ^is_release_build=yes configure.ac; then
     echo "*** is_release_build is set to yes ***"