[CI] Change preview.yml to validate.yml

This change converts the "Preview" job into a "Validate" job. It still
runs for every PR, but it does not deploy to the gh-pages branch.

Fixes patcg-individual-drafts#148

.github/workflows/preview.yml → .github/workflows/validate.yml

@@ -1,17 +1,15 @@
-name: Preview
+name: Validate
 on:
   pull_request:
     paths: ["**.bs"]
 jobs:
-  preview:
-    name: Preview
+  main:
+    name: Validate Spec
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3
       - uses: pythagoraskitty/[email protected]
         with:
           TOOLCHAIN: bikeshed
-          DESTINATION: pr-preview/${{ github.ref }}/index.html
           SOURCE: spec.bs
-          GH_PAGES_BRANCH: gh-pages
           BUILD_FAIL_ON: warning