flatpak-run: unset GIO_EXTRA_MODULES

This variable contains paths to load GIO modules from. For the most
part, they refer to paths outside of the sandbox or if they happen
to be in the sandbox, would contain modules that are incompatible with
the sandbox runtime (ie. different libc).

While I've not found programs that would crash outright, it may cause
unexpected behaviors (eg. Apostrophe not being able to render math in
preview panel).

This variable is set by NixOS for its dependency boxing.

common/flatpak-run.c

@@ -1911,6 +1911,7 @@ static const ExportData default_exports[] = {
  {"GST_INSTALL_PLUGINS_HELPER", NULL},
  {"KRB5CCNAME", NULL},
  {"XKB_CONFIG_ROOT", NULL},
+ {"GIO_EXTRA_MODULES", NULL},
 };
 
 static const ExportData no_ld_so_cache_exports[] = {

doc/flatpak-run.xml

@@ -102,6 +102,7 @@
  <member>XCURSOR_PATH</member>
  <member>KRB5CCNAME</member>
  <member>XKB_CONFIG_ROOT</member>
+ <member>GIO_EXTRA_MODULES</member>
  </simplelist>
  <para>
  Also several environment variables with the prefix "GST_" that are used by gstreamer