Replace `Mutex` use with `-- -M <unique_id>` #2416

Job	Run time
maybe-expedite	5s
test (third-party, bincode, stable, false)	17m 13s
lint	6m 45s
test (third-party, bincode, stable, true)	16m 42s
test (third-party, bincode, nightly, false)	15m 25s
test (third-party, bincode, nightly, true)	15m 24s
test (third-party, postcard, stable, false)	17m 58s
test (third-party, postcard, stable, true)	17m 4s
test (third-party, postcard, nightly, false)	15m 51s
test (third-party, postcard, nightly, true)	15m 59s
test (other, bincode, stable, false)	22m 52s
test (other, bincode, stable, true)	24m 32s
test (other, bincode, nightly, false)	26m 10s
test (other, bincode, nightly, true)	28m 24s
test (other, postcard, stable, false)	23m 13s
test (other, postcard, stable, true)	24m 23s
test (other, postcard, nightly, false)	26m 33s
test (other, postcard, nightly, true)	27m 21s
all-checks	1s
	5h 41m 55s

Provide feedback