Execute cross-build task using PW pool

[cevich]

Previously this task ran using the Cirrus-CI compute service, consuming compute credits. However, since podman is already using a persistent worker pool for CI, it can also be leveraged for use here. Since Mac resources are relatively expensive, it also makes financial sense to re-use infrastructure where possible.

Notes:

• This change also restricts the cross-build task to only running on the default branch. This is necessary because the PW Pool environment is always rolling forward and may not match what was in place at the time a release-branch was created.
• This changes the test from an install + run, to just a build + run. This is required because the task is running as a regular user w/o permissions to install anything system-wide, on a host shared with other CI tasks.

[packit-as-a-service[bot]]

Ephemeral COPR build failed. @containers/packit-build please check.

[cevich]

Leaving this on Draft: I need to update the Mac deployment setup script so it installs gpgme and pkg-config.

[cevich]

force-push: Rebased.

This should be ready to go now.

[rhatdan]

LGTM