refactor(nix-compat/store_path): consistently use SP as type param

We also use S in other places in the same file, but that's for the
string-like references.

SP is now consistently used as the type parameter for StorePath<_> (and
build_output_path) gets support for it).

By being a bit more careful in the order of assignments in nix-compat/
src/derivation, we can nudge the compiler to use the type we want.

Change-Id: Ia7c298e110dff98d3b113d2388674ce9e22b80e8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12590
Reviewed-by: flokli <flokli@flokli.de>
Reviewed-by: Marijan Petričević <marijan.petricevic94@gmail.com>
Tested-by: BuildkiteCI

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

@@ -139,7 +139,7 @@ where
         S: From<&'a str>,
     {
         Ok(Self {
-            name: validate_name(name.as_bytes())?.into(),
+            name: validate_name(name)?.into(),
             digest,
         })
     }