fix(nix-compat/store_path): valid names ⊊ UTF-8

We don't need to validate UTF-8 separately, since valid names are
a strict subset of ASCII, and therefore a strict subset of UTF-8.

Change-Id: I3261bf0efe3480b5b315074efafcf5e47a6c5a65
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10087
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/nix-compat/src/store_path/mod.rs

@@ -261,7 +261,8 @@ pub(crate) fn validate_name(s: &(impl AsRef<[u8]> + ?Sized)) -> Result<&str, Err
         unreachable!();
     }
 
-    Ok(str::from_utf8(s).unwrap())
+    // SAFETY: We permit a subset of ASCII, which guarantees valid UTF-8.
+    Ok(unsafe { str::from_utf8_unchecked(s) })
 }
 
 impl fmt::Display for StorePath {