fix(ci): revert ci-compat-e2e to AWS access keys (#23211)

## Summary

- Drop OIDC auth from `ci-compat-e2e` and run with
`AWS_ACCESS_KEY_ID`/`AWS_SECRET_ACCESS_KEY` instead.
- The OIDC role
(`pipeline-exec-aztecprotocol-aztec-packages-heads-next`) has no
`ec2:RunInstances` policy attached, so spot requests time out and the
on-demand fallback fails with `UnauthorizedOperation`. The job has been
failing on every v4 nightly since #22930; `continue-on-error` for
`-nightly.` tags has masked it.
- Mirrors c3c1371 (#23167), which applied the same workaround to
`ci-release-publish` on this branch.

Example of the failure being patched:
https://github.com/AztecProtocol/aztec-packages/actions/runs/25737242295/job/75580441745

## Test plan

- [ ] CI3 on this PR runs to green (regular `ci` job is unaffected — it
already uses static keys).
- [ ] Apply the `ci-compat-e2e` label here to exercise the compat-e2e
job end-to-end and confirm the EC2 spot/on-demand request succeeds with
the static credentials.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Author: benesjan

.github/workflows/ci3.yml

@@ -298,9 +298,6 @@ jobs:
   # Escape hatch: ci-skip-compat-e2e label makes failures non-blocking on release PRs.
   ci-compat-e2e:
     runs-on: ubuntu-latest
-    permissions:
-      id-token: write
-      contents: read
     needs: [ci]
     if: |
       always()
@@ -320,17 +317,11 @@ jobs:
         with:
           ref: ${{ github.event.pull_request.head.sha || github.sha }}
 
-      - name: Configure AWS credentials (OIDC)
-        uses: aws-actions/configure-aws-credentials@v4
-        with:
-          role-to-assume: ${{ secrets.AWS_OIDC_ROLE_ARN }}
-          aws-region: us-east-2
-          role-session-name: ci3-compat-e2e-${{ github.run_id }}
-          role-duration-seconds: 21600 # 6h – covers AWS_SHUTDOWN_TIME (300 min) + 60 min buffer
-
       - name: Run Backwards Compatibility E2E Tests
         timeout-minutes: 330
         env:
+          AWS_ACCESS_KEY_ID: ${{ secrets.AWS_ACCESS_KEY_ID }}
+          AWS_SECRET_ACCESS_KEY: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
           GITHUB_TOKEN: ${{ secrets.AZTEC_BOT_GITHUB_TOKEN }}
           BUILD_INSTANCE_SSH_KEY: ${{ secrets.BUILD_INSTANCE_SSH_KEY }}
           GCP_SA_KEY: ${{ secrets.GCP_SA_KEY }}