Grab proot from bootstrap zip rather than including its nix path dire… #225

Job	Run time
prepare-n-o-d	1m 42s
prepare-avd (29)	19s
prepare-droidctl	45s
emulate (29, android_integration)	9m 26s
emulate (29, bootstrap_flakes)	8m 21s
emulate (29, bootstrap_channels)	6m 37s
emulate (29, poke_around)	6m 15s
emulate (29, test_channels_uiautomator)	13m 47s
emulate (29, test_channels_shell)	14m 46s
	1h 1m 58s

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