refactor: remove nix from uucore and date, complete source migration

- Remove nix dependency from uucore Cargo.toml
- Remove nix error conversion impls from error.rs, replace with rustix
- Migrate date's clock_settime/clock_getres from nix to rustix::time
- Replace nix dependency with rustix in date's Cargo.toml
- Update test nix features for test utilities that still need it

The nix crate is now completely removed from all source code (src/).
It remains only as a dev-dependency for test code.

tests/uutests/Cargo.toml

@@ -39,7 +39,14 @@ uucore = { workspace = true, features = [
 [target.'cfg(any(target_os = "linux", target_os = "android"))'.dependencies]
 
 [target.'cfg(unix)'.dependencies]
-nix = { workspace = true, features = ["process", "signal", "term", "user"] }
+nix = { workspace = true, features = [
+  "fs",
+  "process",
+  "signal",
+  "socket",
+  "term",
+  "user",
+] }
 rlimit = { workspace = true }
 
 [target.'cfg(all(unix, not(any(target_os = "macos", target_os = "openbsd"))))'.dependencies]