R-CMD-check

add a helper function ctan_mirrors() #276

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Job	Run time
macOS-latest (release)	3m 33s
windows-latest (release)	6m 15s
ubuntu-latest (devel)	3m 6s
ubuntu-latest (release)	2m 25s
ubuntu-latest (oldrel-1)	2m 2s
ubuntu-latest (oldrel-2)	2m 2s
ubuntu-latest (oldrel-3)	2m 30s
ubuntu-latest (oldrel-4)	2m 33s
	24m 26s