gitlab-runner: rename 'preCloneScript' to 'preGetSourcesScript' #17

Job	Run time
install-flake-against-stable	4m 16s
install-against-unstable	4m 10s
install-flake-against-unstable	5m 14s
install-against-stable	4m 38s
test-stable	18m 31s
test-unstable	13m 53s
	50m 42s

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