composefs: fix feature flags of once_cell

We use once_cell because rust-lang/rust#109737

once_cell has a large number of potential locking backends for `no_std`
environments.  We were randomly using `critical-section`, which itself
requires specifying a backend, which we weren't doing.  So far, we've
been protected by this by the fact that we also have once_cell as a
transient dependency (via tempfile).

Fix this to just use `std` so that we get locking via the stdlib.

Signed-off-by: Allison Karlitskaya <[email protected]>

Author: allisonkarlitskaya

crates/composefs/Cargo.toml

@@ -20,7 +20,7 @@ test = []
 anyhow = { version = "1.0.87", default-features = false }
 hex = { version = "0.4.0", default-features = false, features = ["std"] }
 log = { version = "0.4.8", default-features = false }
-once_cell = { version = "1.21.3", default-features = false, features = ["critical-section"] }
+once_cell = { version = "1.21.3", default-features = false, features = ["std"] }
 rustix = { version = "1.0.0", default-features = false, features = ["fs", "mount", "process", "std"] }
 sha2 = { version = "0.10.1", default-features = false, features = ["std"] }
 thiserror = { version = "2.0.0", default-features = false }