GitHub: Don't purge aproxy snap when reclaiming disk space (#14505)

When using this action on the Canonical runners (e.g. MicroCloud) we
cannot purge the aproxy snap as it is a critical component to ensure
connectivity between the runner and GitHub. Removing it will cause the
runner to hang up.

See
[this](https://github.com/canonical/microcloud/actions/runs/11934150325/job/33262745369)
job for an example. The error message is something like the following:

```
The self-hosted runner: openstack-amd64-large-ps6-3-c9077065b675 lost communication with the server. Verify the machine is running and has a healthy network connection. Anything in your workflow that terminates the runner process, starves it for CPU/Memory, or blocks its network access can cause this error.
```

.github/actions/reclaim-disk-space/action.yml

@@ -9,8 +9,9 @@ runs:
       run: |
         set -eux
 
-        # Purge snaps, if any
-        for s in $(snap list | awk '!/^(Name|core|snapd)/ {print $1}'); do
+        # Purge snaps, if any.
+        # The Canonical runners use aproxy for connectivity.
+        for s in $(snap list | awk '!/^(Name|core|snapd|aproxy)/ {print $1}'); do
           sudo snap remove --purge "${s}" || true
         done