Fix prelude finding for direct driver execution. (#3894)

This is just to be able to directly run the driver (e.g., `bazel run
//toolchain/driver:carbon compile foo.carbon`); it shouldn't affect
anything else.

toolchain/testing/file_test.cpp

@@ -31,7 +31,7 @@ class ToolchainFileTest : public FileTestBase {
            llvm::raw_pwrite_stream& stderr) -> ErrorOr<RunResult> override {
     CARBON_RETURN_IF_ERROR(AddFile(fs, "core/prelude.carbon"));
 
-    Driver driver(fs, stdout, stderr);
+    Driver driver(fs, "", stdout, stderr);
     auto driver_result = driver.RunCommand(test_args);
 
     RunResult result{