fix: use GitHub URL for erlef/setup-beam action

Forgejo doesn't have this action in its registry, so we need to
use the full GitHub URL to fetch it from GitHub's action registry.

.forgejo/workflows/build.yaml

@@ -36,7 +36,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Set up Elixir
-        uses: erlef/setup-beam@v1
+        uses: https://github.com/erlef/setup-beam@v1
         with:
           otp-version: '27.1'
           elixir-version: '1.17.3'