coordinator: Limit git clone parallelism #630

Job	Run time
test-e2e (almond-sh/almond, mill 0.11)	10m 27s
test-e2e (scalameta/munit, sbt)	1m 54s
test-build	4m 15s
test-e2e (com-lihaoyi/mill, mill 1.0)	7m 45s
test-e2e (blackdoor/jose, mill 0.9)	3m 48s
test-e2e (virtuslab/scala-cli, mill 0.12)	6m 21s
test-e2e (kordyjan/pytanie, scala-cli)	2m 27s
test-e2e (barnardb/tinyplate, mill 0.10)	2m 31s
	39m 28s

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