Fix compilation of nix-build on Windows

src/nix-build/nix-build.cc

@@ -536,7 +536,7 @@ static void main_nix_build(int argc, char * * argv)
             env["__ETC_PROFILE_SOURCED"] = "1";
         }
 
-        env["NIX_BUILD_TOP"] = env["TMPDIR"] = env["TEMPDIR"] = env["TMP"] = env["TEMP"] = tmpDir.path();
+        env["NIX_BUILD_TOP"] = env["TMPDIR"] = env["TEMPDIR"] = env["TMP"] = env["TEMP"] = tmpDir.path().string();
         env["NIX_STORE"] = store->storeDir;
         env["NIX_BUILD_CORES"] = std::to_string(settings.buildCores);