Modify concurrency setting

It was added to potentially cancel superseded running tasks, but it was
also making the `niri` build wait until the `gnome` build was done. My
bad.

.gitea/workflows/build.yml

@@ -27,12 +27,6 @@ env:
     https://davejansen.dev/${{ github.repository_owner }}/${{
     github.event.repository.name }}
 
-concurrency:
-  group:
-    ${{ github.workflow }}-${{ github.ref || github.run_id }}-${{
-    inputs.brand_name}}-${{ inputs.stream_name }}
-  cancel-in-progress: true
-
 jobs:
   build_push:
     name: Build and push image
@@ -47,6 +41,12 @@ jobs:
       packages: write
       id-token: write
 
+    concurrency:
+      group:
+        ${{ github.workflow }}-${{ github.ref || github.run_id }}-${{
+        matrix.variant }}
+      cancel-in-progress: true
+
     steps:
       - name: Prepare environment
         run: |