Clean up registry username shenanigans a bit.

.gitea/workflows/build.yaml

@@ -14,8 +14,7 @@ on:
   workflow_dispatch:
 
 env:
-  REGISTRY: ${{ REGISTRY || 'davejansen.dev' }}
-  REGISTRY_USER: ${{ REGISTRY_USER || github.actor }}
+  REGISTRY: ${{ vars.REGISTRY || 'davejansen.dev' }}
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref || github.run_id }}-${{
@@ -58,7 +57,7 @@ jobs:
         uses: docker/login-action@v3
         with:
           registry: ${{ env.REGISTRY }}
-          username: ${{ env.REGISTRY_USER }}
+          username: ${{ github.actor }}
           password: ${{ secrets.REGISTRY_TOKEN }}
 
       - name: Build and push image