release.yml: Run on every pull request

Apparently dist is of the opinion that it needs to run on every pull request.

Author: davidlattimore

.github/workflows/release.yml

@@ -39,6 +39,7 @@ permissions:
 # If there's a prerelease-style suffix to the version, then the release(s)
 # will be marked as a prerelease.
 on:
+  pull_request:
   push:
     tags:
       - '**[0-9]+.[0-9]+.[0-9]+*'