Only run upgrade rust workflow on main repo

Fixes #344

Author: LegNeato

.github/workflows/rust-toolchain-upgrade.yml

@@ -13,6 +13,7 @@ permissions:
 jobs:
   check-and-upgrade:
     runs-on: ubuntu-latest
+    if: github.repository == 'Rust-GPU/rust-gpu'
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4