ci: footprint: Use "concurrency" to cancel previous runs

This commit adds a concurrency group to the workflow in order to ensure
that only one instance of the workflow runs for an event-ref
combination.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

.github/workflows/footprint.yml

@@ -2,19 +2,14 @@ name: Footprint Delta
 
 on: pull_request
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event_name }}-${{ github.head_ref || github.ref }}
+  cancel-in-progress: true
+
 jobs:
-  footprint-cancel:
-    runs-on: ubuntu-20.04
-    if: github.repository == 'zephyrproject-rtos/zephyr'
-    steps:
-      - name: Cancel Previous Runs
-        uses: styfle/cancel-workflow-action@0.11.0
-        with:
-          access_token: ${{ github.token }}
   footprint-delta:
     runs-on: ubuntu-20.04
     if: github.repository == 'zephyrproject-rtos/zephyr'
-    needs: footprint-cancel
     container:
       image: ghcr.io/zephyrproject-rtos/ci:v0.24.5
       options: '--entrypoint /bin/bash'
@@ -24,11 +19,6 @@ jobs:
       ZEPHYR_SDK_INSTALL_DIR: /opt/toolchains/zephyr-sdk-0.15.1
       ZEPHYR_TOOLCHAIN_VARIANT: zephyr
     steps:
-      - name: Cancel Previous Runs
-        uses: styfle/cancel-workflow-action@0.11.0
-        with:
-          access_token: ${{ github.token }}
-
       - name: Apply container owner mismatch workaround
         run: |
           # FIXME: The owner UID of the GITHUB_WORKSPACE directory may not