Fix compatibility with old Cargo #2688

Job	Run time
miri	1m 29s
test (no rustup)	2m 59s
test (stable)	9m 29s
test (1.26)	11m 56s
test (beta)	9m 15s
test (1.30)	11m 10s
test (nightly)	7m 49s
test (1.31)	11m 55s
test (nightly, macos-11)	9m 31s
test (1.36)	11m 32s
test (nightly, windows-2019)	12m 16s
test (1.39)	11m 21s
test (1.41)	11m 4s
deny / deny	17s
msrv / msrv	6m 41s
release-dry-run / prepare	9s
tidy / tidy	1m 23s
release-dry-run / build (aarch64-unknown-linux-gnu)	1m 51s
release-dry-run / build (aarch64-unknown-linux-musl)	1m 56s
release-dry-run / build (aarch64-apple-darwin, macos-11)	1m 51s
release-dry-run / build (aarch64-pc-windows-msvc, windows-2019)	3m 47s
release-dry-run / build (x86_64-unknown-linux-gnu)	1m 23s
release-dry-run / build (x86_64-unknown-linux-musl)	1m 26s
release-dry-run / build (x86_64-apple-darwin, macos-11)	1m 49s
release-dry-run / build (x86_64-pc-windows-msvc, windows-2019)	3m 4s
release-dry-run / build (x86_64-unknown-freebsd)	1m 49s
release-dry-run / build (universal-apple-darwin, macos-11)	2m 11s
	2h 31m 23s

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