ci: Try other syntax for concurrency key (#2935)

After #2932 that [immediately failed][build] on the main branch so this
tries a different key to see if it'll work...

[build]: https://github.com/bytecodealliance/wasmtime/actions/runs/872766013/workflow

.github/workflows/main.yml

@@ -12,7 +12,7 @@ defaults:
 # Cancel any in-flight jobs for the same PR/branch so there's only one active
 # at a time
 concurrency:
-  group: ${{ github.head_ref }}
+  group: ${{ github.workflow }}-${{ github.ref }}
   cancel-in-progress: true
 
 jobs: