Merge bitcoin-core/secp256k1#1403: ci, gha: Ensure only a single workflow processes github.ref at a time

b0886fd35c ci, gha: Ensure only a single workflow processes `github.ref` at a time (Hennadii Stepanov)

Pull request description:

ACKs for top commit:
  real-or-random:
    ACK b0886fd35c
  jonasnick:
    ACK b0886fd35c

Tree-SHA512: 8edda9259fc07bda3a35286ab97238b2f2749fbc629030da52e5a352988e5562cf62255c7d4917b7f0c8dbc9a0bd3a36b5e725d3d5a4c635ae8239faef829d1b

.github/workflows/ci.yml

@@ -7,6 +7,10 @@ on:
     tags-ignore:
       - '**'
 
+concurrency:
+  group: ${{ github.ref }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
+
 env:
   ### compiler options
   HOST: