fetchToStore(): Don't always respect settings.readOnlyMode

It's now up to the caller whether readOnlyMode should be applied. In
some contexts (like InputScheme::fetch()), we always need to fetch.

src/libfetchers/fetch-to-store.hh

@@ -8,12 +8,15 @@
 
 namespace nix {
 
+enum struct FetchMode { DryRun, Copy };
+
 /**
  * Copy the `path` to the Nix store.
  */
 StorePath fetchToStore(
     Store & store,
     const SourcePath & path,
+    FetchMode mode,
     std::string_view name = "source",
     ContentAddressMethod method = FileIngestionMethod::Recursive,
     PathFilter * filter = nullptr,