CI: Add FreeBSD build tests for staged commits

[kinkie]

Use a GitHub-hosted VM to create FreeBSD test environment.

We have not enabled ccache optimization for FreeBSD tests because we
do not know how to copy updated ccache files from VM back into runner.

[kinkie]

test run: https://github.com/kinkie/squid/actions/runs/11882745983
It is to be verified if "vmactions" actions are authorized to run in Squid,
I can't test it on my own repository

[kinkie]

ccache doesn't work, ccache-action is not compatible with vmaction

I saw ccache being used to build Squid, and some ccache-related files being copied to and from VM, but I am guessing that is not enough. Just for my own edification (this is not a change request), do you know what makes the two actions incompatible?

In https://github.com/kinkie/squid/actions/runs/11917155891/job/33211530535 , ccache stats show 0 across the board.
This is because in the 'Test builds' stage, the cache directory is not copied back from the VM to the worker environment, therefore it is not saved across builds; in other words, at this time it's a waste of resources

[rousskov]

ccache doesn't work, ccache-action is not compatible with vmaction

In https://github.com/kinkie/squid/actions/runs/11917155891/job/33211530535 , ccache stats show 0 across the board. This is because in the 'Test builds' stage, the cache directory is not copied back from the VM to the worker environment

#[group]Initialize files in VM
sending incremental file list
squid/squid/.ccache/0/0/stats
... more ccache files follow ...

##[group]Copy files back from the VM
receiving incremental file list
... no ccache files at all on this list ...

Thank you for this research. I added the corresponding text to PR description.

[kinkie]

Test run as of now: https://github.com/kinkie/squid/actions/runs/11937092476/job/33272288193

[kinkie]

Thanks for approving!

[kinkie]

Trying to land this; if the actions need to be approved, I'll authorize them as needed.