libexpr: Use Store::requireStoreObjectAccessor instead or toRealPath in fetch

This forces the code to go through proper abstractions instead of the raw filesystem
API.

This issue is evident from this reproducer:

nix eval --expr 'builtins.fetchurl { url = "https://example.com"; sha256 = ""; }' --json --eval-store "dummy://?read-only=false"

error:
       … while calling the 'fetchurl' builtin
         at «string»:1:1:
            1| builtins.fetchurl { url = "https://example.com"; sha256 = ""; }
             | ^

       error: opening file '/nix/store/r4f87yrl98f2m6v9z8ai2rbg4qwlcakq-example.com': No such file or directory

src/libexpr/primops/fetchTree.cc

@@ -588,7 +588,11 @@ static void fetch(
 
     if (expectedHash) {
         auto hash = unpack ? state.store->queryPathInfo(storePath)->narHash
-                           : hashFile(HashAlgorithm::SHA256, state.store->toRealPath(storePath));
+                           : hashPath(
+                                 {state.store->requireStoreObjectAccessor(storePath)},
+                                 FileSerialisationMethod::Flat,
+                                 HashAlgorithm::SHA256)
+                                 .hash;
         if (hash != *expectedHash) {
             state
                 .error<EvalError>(