`gh-actions/cache/prime`: Use heredoc for creating command #4457

Job	Run time
gh-actions (20.x, appauth)	25s
gh-actions (20.x, dispatch)	23s
gh-actions (20.x, github/checks)	26s
gh-actions (20.x, github/mutex)	25s
gh-actions (20.x, github/script/run)	19s
gh-actions (20.x, hashfiles)	18s
gh-actions (20.x, jq)	20s
gh-actions (20.x, retest)	29s
gh-actions (20.x, torun)	18s
	3m 23s

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