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/libexpr/primops.cc

@@ -2228,7 +2228,14 @@ static void addPath(
             });
 
         if (!expectedHash || !state.store->isValidPath(*expectedStorePath)) {
-            auto dstPath = fetchToStore(*state.store, path.resolveSymlinks(), name, method, filter.get(), state.repair);
+            auto dstPath = fetchToStore(
+                *state.store,
+                path.resolveSymlinks(),
+                settings.readOnlyMode ? FetchMode::DryRun : FetchMode::Copy,
+                name,
+                method,
+                filter.get(),
+                state.repair);
             if (expectedHash && expectedStorePath != dstPath)
                 state.error<EvalError>(
                     "store path mismatch in (possibly filtered) path added from '%s'",