ci: add dry-run publish check to PRs (#3791)

- Adds `deno publish --dry-run` step to CI (ubuntu, v2.x only)
- Catches lockfile mismatches and other publish issues in PRs before
they hit main
- Prevents the recurring problem of releases failing to publish after
merge

Author: bartlomieju

.github/workflows/ci.yml

@@ -78,3 +78,7 @@ jobs:
       - name: Check links
         if: startsWith(matrix.os, 'ubuntu') && matrix.deno == 'v2.x'
         run: deno task check:links
+
+      - name: Dry-run publish
+        if: startsWith(matrix.os, 'ubuntu') && matrix.deno == 'v2.x'
+        run: deno publish --dry-run