quick workaround for CI issue with libc & MSRV (Rust 1.56) #2967

Job	Run time
cargo +1.36 build (futures-{core, io, sink})	37s
cargo test (ubuntu-latest)	2m 48s
cargo test (macos-latest)	5m 56s
cargo clippy	16s
cargo +stable build	30s
cargo doc	17s
cargo test (windows-latest)	4m 36s
cargo +beta build	37s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 28s
cargo +nightly build	33s
cargo hack check --feature-powerset	2m 23s
cargo fmt	5s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 9s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 27s
cargo minimal-versions build	22s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 30s
cargo miri test	14m 6s
cargo build --target thumbv7m-none-eabi	23s
cargo build --target thumbv6m-none-eabi	23s
cargo test -Z sanitizer=address	3m 28s
cargo +1.56 build	1m 10s
cargo test -Z sanitizer=memory	5m 26s
cargo test -Z sanitizer=thread	2m 27s
cargo bench	1m 1s
	1h 6m 58s

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