When the portal's Spawn method is used with the environment cleared,
it's very likely that the "flatpak run" that ends up being run will be
in an environment without UTF-8 support.
If one of the files or directories we try to expose to the sub-sandbox
contains UTF-8/non-ASCII characters, then "flatpak run" would fail with:
error: Invalid byte sequence in conversion input
This is caused by GOption trying to parse the --filesystem option for
flatpak, as, when using the G_OPTION_ARG_CALLBACK argument type, GOption
will split the option name from its value, and try to convert the value
to UTF-8. Which will fail because there's no UTF-8.
It won't however do that if we tell the option parser that the value is
a filename using G_OPTION_FLAG_FILENAME, so set it.
e67847e253