Refine tag matching in the release workflow (#817)

Improve the `on.push.tags` entry in `.github/workflows/release.yml` to
use a regular expression that matches only non-pre-release semantic
version tags. This should prevent it from releasing a prerelease on
PGXN. It will need refining should the desire arise to put prerelease
versions on PGXN.

Author: theory

.github/workflows/release.yml

@@ -1,7 +1,7 @@
 name: Release
 on:
   push:
-    tags: [v*]
+    tags: ["v[0-9]+.[0-9]+.[0-9]+"]
 jobs:
   release:
     name: Release on PGXN