scripts: allow to configure `PROXMOX_GIT_URL` #138

Job	Run time
docker-build (ubuntu-24.04-arm, arm64)	28m 27s
client-build (ubuntu-24.04-arm, arm64)	5m 14s
client-build (ubuntu-24.04, amd64)	4m 53s
client-build (ubuntu-24.04-arm, arm32, true)	2m 51s
docker-build (ubuntu-24.04, amd64)	35m 11s
manifest	13s
	1h 16m 49s

Provide feedback