Install proxy cert on GitHub-hosted Ubuntu runners #6788

Job	Run time
Ruby analysis (ubuntu-latest, nightly-latest)	49s
Ruby analysis (ubuntu-latest, default)	49s
Ruby analysis (ubuntu-latest, linked)	48s
Ruby analysis (macos-latest, nightly-latest)	1m 32s
Ruby analysis (macos-latest, linked)	1m 32s
Ruby analysis (macos-latest, default)	58s
	6m 28s

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