Feature request: migrate2rocky.sh mirrors pre-check

[mjtrangoni]

Hi @pajamian,

Running migrate2rocky.sh behind a firewall is kind of complicated because you could end up frequently with a broken installation, as the script has no "persistence" state, like I read in other issues.

What about checking for working mirrors in beforehand, and blocking the script execution if they are failing?
Maybe also adding an option to allow hardcoding a local repository, or an external fixed one would be nice.

Thank you!

[m41kc0d3]

please see #92
and my fork https://github.com/m41kc0d3/rocky-tools/tree/set-mirror-url

we used it internally for many migrations