CLD-7473 Limit parallelism for full E2E tests (mattermost#26741)

* Limit parallelism for full E2E tests

* Add temporary changes for testing

* Better concurrency spec

* Do not consider github.ref for concurrency grouping

* Further improve the concurrency grouping

* Remove testing lines

Author: mvitale1989

.github/workflows/e2e-fulltests-ci.yml

@@ -29,6 +29,10 @@ on:
           - NONE
         default: NONE
 
+concurrency:
+  group: "${{ github.workflow }}-${{ inputs.PR_NUMBER || inputs.ref }}"
+  cancel-in-progress: true
+
 jobs:
   resolve-ref:
     runs-on: ubuntu-22.04