Fix: workflow repository name configurable via GIT_REPO_NAME

[MateoGreil]

Hi!

Here is a little fix on workflow, for fork repo.

The install script hard-coded the GitHub repository name as RPi-Jukebox-RFID when building the source download URL. Installing from a fork whose repository was renamed therefore produced a 404, the empty tarball broke 'tar xz' and the install aborted with 'Couldn't find git download.'

Allow GIT_REPO_NAME to be overridden (defaulting to RPi-Jukebox-RFID) and pass it through the CI install-script tests from the repository name, so the suite works on renamed forks.

Workflow error before: https://github.com/MateoGreil/phoniebox/actions/runs/27874252323
Workflow running now: https://github.com/MateoGreil/phoniebox/actions/runs/27875331806

[MateoGreil]

I can't make run again the CI, but the actual fails looks like coming from a unavailable services. I guess it will success after a re-run

#15 59.15 W: Failed to fetch http://raspbian.raspberrypi.org/raspbian/dists/bullseye/rpi/binary-armhf/Packages  Cannot initiate the connection to raspbian.raspberrypi.org:80 (2a00:1098:0:80:1000:75:0:3). - connect (101: Network is unreachable) [IP: 93.93.128.193 80]

--2026-06-20 18:03:05--  (try:20)  http://abyz.me.uk/lg/lg.zip
Connecting to abyz.me.uk (abyz.me.uk)|91.238.160.173|:80... failed: Connection timed out.
Giving up.