scripts: use git.savannah.gnu.org mirrors #34

Job	Run time
Build base image (ubuntu-latest)	20m 12s
Build base image (ubuntu-24.04-arm)	27s
Build target base image (win64)	57m 52s
Build target base image (linux64)	55m 6s
Build target base image (linuxarm64)	55m 55s
Build target-variant image (win64, gpl 7.1)	1h 16m 39s
Build target-variant image (linuxarm64, gpl 7.1)	1h 1m 49s
Build target-variant image (linux64, gpl 7.1)	1h 5m 14s
Build ffmpeg (linux64, gpl 7.1)	11m 10s
Build ffmpeg (win64, gpl 7.1)	11m 12s
Build ffmpeg (linuxarm64, gpl 7.1)	10m 55s
Publish release	0s
	7h 6m 31s

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