ci: stop running the matrix twice per pull request

The push and pull_request triggers both matched every branch, so each PR
kicked off two identical runs of lint, the three-OS test matrix and the
soulfind e2e job. Push now only covers master; branches are covered by the
pull_request trigger. Adds a concurrency group so a follow-up push cancels
the superseded run instead of queueing behind it.

.github/workflows/ci.yml

@@ -1,11 +1,17 @@
 name: CI
 
+# Branch pushes are covered by the pull_request trigger. Listening to both for
+# every branch ran the whole matrix twice per PR.
 on:
   push:
-    branches: ['**']
+    branches: [master]
   pull_request:
     branches: ['**']
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 env:
   CARGO_TERM_COLOR: always
   RUST_BACKTRACE: 1