chore: gitignore nix-direnv files #1362

Job	Run time
changed_paths	5s
cargo test (host)	4m 48s
clippy	2m 59s
cargo check (host)	3m 42s
build boot image (x86_64)	3m 17s
Loom tests (cordyceps)	1m 31s
docs	4m 23s
rustfmt	15s
cargo test (cross x64)	4m 19s
Miri tests (codyceps)	2m 4s
Loom tests (maitake)	4m 31s
Tests (maitake, no-default-features)	1m 39s
Loom tests (mycelium-util)	53s
	34m 26s

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