refactor(tvix/nix-compat): have helpers interact with StorePathRef

In most case, we don't actually need an owned `StorePath` struct, but a
`StorePathRef<'_>` is sufficient. The lifetime is only due to it holding
onto the name, but that one is mostly left untouched.

`Derivation::calculate_derivation_path` still needs to return
`StorePath`, as its name has a `.drv` appended.

Change-Id: Ie0d52f369d785711bb0658ea2b0bd2617fd9f45e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10389
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: edef <edef@edef.eu>

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

@@ -115,6 +115,7 @@ impl Derivation {
         };
 
         build_text_path(name, self.to_aterm_bytes(), references)
+            .map(|s| s.to_owned())
             .map_err(|_e| DerivationError::InvalidOutputName(name.to_string()))
     }