Don't build libmongoc version in pull requests

Author: alcaeus

.evergreen/config.yml

@@ -29,7 +29,7 @@ post:
 github_pr_aliases: &github_pr_aliases
   # Always test all builds for consistency
   - variant_tags: ["pr build"]
-    task_tags: ["pr"]
+    task_tags: ["pr !build-libmongoc"]
   # Run all tasks in PR variants for PHP 8.3 (excluding MongoDB latest)
   - variant_tags: ["pr php8.3"]
     task_tags: ["!latest"]