Make `HostTargetTools` reference arguments `const` #356

Job	Run time
build-installer-iso-job	2m 40s
build-metapackage-deb-job	7m 2s
build-uboot-qemu-job	3m 36s
build-cuttlefish-deb-job / build-arm64-cuttlefish-deb-job	1h 11m 20s
test-install-metapackage-deb-job	9m 29s
test-install-cuttlefish-deb-job	11m 1s
	1h 45m 8s

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