scripts: allow to configure `PROXMOX_GIT_URL` (#102) #139

Job	Run time
client-build (ubuntu-24.04-arm, arm32, true)	2m 51s
docker-build (ubuntu-24.04, amd64)	32m 58s
docker-build (ubuntu-24.04-arm, arm64)	28m 57s
client-build (ubuntu-24.04-arm, arm64)	4m 41s
client-build (ubuntu-24.04, amd64)	5m 10s
manifest	8s
	1h 14m 45s

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