Reusable rock PR workflow fails on Noble when `apt update` hits Microsoft package sources

## Summary
The reusable `rock-pull-request` fails during environment setup before project build/test steps run.

A [failing run](https://github.com/canonical/temporal-rocks/actions/runs/28900377209/job/85735529051#step:3:72) from [PR #20](https://github.com/canonical/temporal-rocks/pull/20) shows `apt update` failing on Microsoft package sources on Ubuntu Noble:

- `https://packages.microsoft.com/repos/azure-cli/dists/noble/InRelease`
- `https://packages.microsoft.com/ubuntu/24.04/prod/dists/noble/InRelease`

Errors from the job log include:

- `Clearsigned file isn't valid, got 'NOSPLIT'`
- `The repository 'https://packages.microsoft.com/repos/azure-cli noble InRelease' is no longer signed.`
- `failed to update apt cache: failed to update apt package lists: exit status 100`

## Reproduction / Evidence
Affected run:
- https://github.com/canonical/temporal-rocks/actions/runs/28900377209/job/85735529051?pr=20

Relevant log excerpts:
```text
E: Failed to fetch https://packages.microsoft.com/repos/azure-cli/dists/noble/InRelease Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)
E: The repository 'https://packages.microsoft.com/repos/azure-cli noble InRelease' is no longer signed.
E: Failed to fetch https://packages.microsoft.com/ubuntu/24.04/prod/dists/noble/InRelease Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)
time=2026-07-07T21:38:05.150Z level=ERROR msg="concierge failed" error="failed to update apt cache: failed to update apt package lists: exit status 100"
```

## Proposed fix (by copilot)
Before the workflow runs any `apt update` or bootstrap step that refreshes the apt cache, disable or remove the Microsoft package sources if they are not required for the job.

Example mitigation:

```bash
sudo rm -f /etc/apt/sources.list.d/azure-cli.list
sudo rm -f /etc/apt/sources.list.d/microsoft-prod.list
sudo apt-get update
```

Start with the reusable `rock-pull-request` workflow and the failing PR #20 run, then inspect the environment setup immediately before its `apt update` or bootstrap step. Done means Microsoft package sources no longer cause the Noble setup to fail and the workflow proceeds to project build and test steps.

Written by the indexing model from the issue text.