chore(ci): optimize disk usage in arm run (#8564)

This was creating unnecessary ARM disks per user

.github/workflows/ci-arm.yml

@@ -25,7 +25,7 @@ jobs:
   setup:
     uses: ./.github/workflows/setup-runner.yml
     with:
-      username: ${{ github.event.pull_request.user.login || github.actor }}
+      username: master
       runner_type: builder-arm
     secrets: inherit
 
@@ -49,24 +49,27 @@ jobs:
 
   build-images:
     needs: [setup, changes]
-    runs-on: ${{ github.event.pull_request.user.login || github.actor }}-arm
+    # Note we don't put the 'if' here as that would also apply to dependent jobs, instead we just copy it into each step
+    runs-on: master-arm
     steps:
       # permission kludge before checkout, see https://github.com/actions/checkout/issues/211#issuecomment-611986243
       - run: sudo chown -R $USER:$USER /home/ubuntu/
+        if: needs.changes.outputs.build-images == 'true'
       - uses: actions/checkout@v4
         with: { ref: "${{ env.GIT_COMMIT }}" }
       - uses: ./.github/ci-setup-action
+        if: needs.changes.outputs.build-images == 'true'
         with:
           concurrency_key: build-images-arm
       - name: "Push Build Images If Changed"
-        if: ${{ needs.changes.outputs.build-images }}
+        if: needs.changes.outputs.build-images == 'true'
         timeout-minutes: 40
         run: |
           earthly-ci --push ./build-images/+build
 
   build:
     needs: [build-images]
-    runs-on: ${{ github.event.pull_request.user.login || github.actor }}-arm
+    runs-on: master-arm
     steps:
       # permission kludge before checkout, see https://github.com/actions/checkout/issues/211#issuecomment-611986243
       - run: sudo chown -R $USER:$USER /home/ubuntu/
@@ -84,7 +87,7 @@ jobs:
   # all the non-bench end-to-end integration tests for aztec
   e2e:
     needs: [build]
-    runs-on: ${{ github.event.pull_request.user.login || github.actor }}-arm
+    runs-on: master-arm
     steps:
       # permission kludge before checkout, see https://github.com/actions/checkout/issues/211#issuecomment-611986243
       - run: sudo chown -R $USER:$USER /home/ubuntu/