Added Github Workflow concurrency.

Author: diegotori

.github/workflows/ci.yml

@@ -11,6 +11,10 @@ on:
       - '**.md'
   workflow_dispatch:
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   # Does a sanity check that packages at least pass analysis on the N-1
   # versions of Flutter stable if the package claims to support that version.