ci: fix cross-compile sync issues

[bandoti]

During the Linux cross-compile CI setup there seem to be issues syncing dependencies, causing unnecessary build failures and delays in CI success. This PR is an attempt to address that issue.

[bandoti]

@slaren I am going to submit a separate PR tomorrow to (for now) remove the cross-compiling from the CI. I don't currently have time to get this one working, so that will be a workaround.

[bandoti]

@slaren It looks like the solution was hiding in plain sight! The apt-get update was only failing when bad repository URLs appeared at the end of the output, which is why failures were somewhat random. So, I think it's good to go now after suppressing those failures.

One thing I changed is how the ports repositories are added. Instead of mangling the default azure repositories (which only applies to Github runners), I opted to create a more realistic cross-compile example by directly adding the Ubuntu ports repos alongside the existing ones. This way folks could perform the same steps on a local Ubuntu system without issue.

[slaren]

Nice! Minor nit, the fetch-depth: 0 in the checkout actions shouldn't be necessary here, that's only used in the releases so that the build number can be calculated correctly.

[bandoti]

Fixed the fetch depth will merge once build finishes. 😊