refactor(nix-compat/store_path): make StorePath generic on S

Similar to how cl/12253 already did this for `Signature`, we apply the
same logic to `StorePath`.

`StorePathRef<'a>'` is now a `StorePath<&'a str>`, and there's less
redundant code for the two different implementation.

`.as_ref()` returns a `StorePathRef<'_>`, `.to_owned()` gives a
`StorePath<String>` (for now).

I briefly thought about only publicly exporting `StorePath<String>`
as `StorePath`, but the diff is not too large and this will make it
easier to gradually introduce more flexibility in which store paths to
accept.

Also, remove some silliness in `StorePath::from_absolute_path_full`,
which now doesn't allocate anymore.

Change-Id: Ife8843857a1a0a3a99177ca997649fd45b8198e6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12258
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: Connor Brewster <cbrewster@hey.com>

tvix/glue/src/builtins/derivation.rs

@@ -568,7 +568,7 @@ pub(crate) mod derivation_builtins {
             let blob_digest = blob_writer.close().await?;
             let ca_hash = CAHash::Text(Sha256::digest(&content).into());
 
-            let store_path =
+            let store_path: StorePathRef =
                 build_ca_path(name.to_str()?, &ca_hash, content.iter_ctx_plain(), false)
                     .map_err(|_e| {
                         nix_compat::derivation::DerivationError::InvalidOutputName(