Parallelize CI with caching #56

Job	Run time
build_qemu_core	21s
build_nrf52_usb_crates_and_upload_fw (puzzle-fw)	50s
build_and_test_nrf52_crates (nrf52-code/usb-lib-solutions/complete)	16s
build_nrf52_usb_crates (nrf52-code/boards/dk)	34s
build_and_test_nrf52_crates (nrf52-code/usb-lib-solutions/get-descriptor-config)	21s
build_nrf52_usb_crates (nrf52-code/boards/dk-solution)	39s
build_and_test_nrf52_crates (nrf52-code/usb-lib-solutions/get-device)	16s
build_nrf52_usb_crates_and_upload_fw (loopback-fw)	48s
check_fmt	7s
build_nrf52_usb_crates (nrf52-code/boards/dongle)	37s
build_and_test_nrf52_crates (nrf52-code/usb-lib-solutions/set-config)	16s
build_nrf52_usb_crates (nrf52-code/radio-app)	40s
build_nrf52_usb_crates (nrf52-code/usb-app)	41s
check_templates	22s
build_nrf52_usb_crates (nrf52-code/usb-app-solutions)	42s
test_and_build_mdbook	21s
test_exercise_solutions_and_examples	30s
build_nrf52_usb_crates (nrf52-code/consts)	9s
test_exercise_solutions_standalone_crates (exercise-solutions/connected-mailbox)	15s
test_exercise_solutions_standalone_crates (exercise-solutions/multi-threaded-mailbox)	11s
deploy	10s
	9m 6s

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