set a repository for install.packages in the rstan workflow #282

Job	Run time
downstream (windows-latest, 4.2)	5m 12s
downstream (windows-latest, release)	2m 55s
ubuntu-latest (release)	3m 7s
macOS-latest (release)	3m 15s
windows-latest (release)	7m 11s
ubuntu-24.04-arm (release)	3m 1s
windows-11-arm (release)	8m 32s
windows-latest (4.2)	10m 9s
downstream (windows-11-arm, release)	5m 52s
	49m 14s

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