LibCore: Use new format functions in some places.

Author: https://github.com/asynts
Commit: c9ca897a45
Pull-request: https://github.com/SerenityOS/serenity/pull/3776
Reviewed-by: https://github.com/alimpfard

Libraries/LibCore/Command.cpp

@@ -110,7 +110,7 @@ String command(const String& program, const Vector<String>& arguments, Optional<
 
     if (WEXITSTATUS(wstatus) != 0) {
 #    ifdef DBG_FAILED_COMMANDS
-        dbg() << "command failed. stderr: " << read_all_from_pipe(stderr_pipe);
+        dbgln("command failed. stderr: {}", read_all_from_pipe(stderr_pipe));
 #    endif
         return {};
     }