Fix input_files in projects/firefox-l10n/config

In input_files in projects/firefox-l10n/config we are doing this:

      my $locales = project_config($project, 'var/locales', $options);
      foreach my $locale (@$locales) {
        $locale = process_template($project, $locale, '.');

In this foreach loop, $locale is pointing to the real string from var/locales and not a copy, and we are updating it with the output from process_template. This means that the template will be processed the first time the function is called (for example for a linux-x86_64 build) and the processed value from the first call reused on subsequent calls (for example a macos build, where the ja locale is supposed to be different).

This is what caused the reproducibility issue in !591 (comment 2857042)