[gnome-builder] flatpak: give flatpak subprocess access to cache dir

[Christian Hergert <chergert src gnome org>]

commit f1303cd00c9e6452c77293a1919c6ddc10644ba5
Author: Christian Hergert <chergert redhat com>
Date:   Sun Feb 11 19:54:52 2018 -0800

    flatpak: give flatpak subprocess access to cache dir
    
    We need this when running the build layer and may not be able to depend
    on layers beneath us to enable it.

 src/plugins/flatpak/gbp-flatpak-runtime.c | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/src/plugins/flatpak/gbp-flatpak-runtime.c b/src/plugins/flatpak/gbp-flatpak-runtime.c
index 5a86f03a9..84758b158 100644
--- a/src/plugins/flatpak/gbp-flatpak-runtime.c
+++ b/src/plugins/flatpak/gbp-flatpak-runtime.c
@@ -200,12 +200,15 @@ gbp_flatpak_runtime_create_launcher (IdeRuntime  *runtime,
         {
           g_autofree gchar *filesystem_option_src = NULL;
           g_autofree gchar *filesystem_option_build = NULL;
+          g_autofree gchar *filesystem_option_cache = NULL;
           g_autofree gchar *build_dir_option = NULL;
 
           filesystem_option_src = g_strdup_printf ("--filesystem=%s", project_path);
           filesystem_option_build = g_strdup_printf ("--filesystem=%s", builddir);
+          filesystem_option_cache = g_strdup_printf ("--filesystem=%s/gnome-builder", g_get_user_cache_dir 
());
           build_dir_option = g_strdup_printf ("--build-dir=%s", builddir);
           ide_subprocess_launcher_push_argv (ret, "--nofilesystem=host");
+          ide_subprocess_launcher_push_argv (ret, filesystem_option_cache);
           ide_subprocess_launcher_push_argv (ret, filesystem_option_src);
           ide_subprocess_launcher_push_argv (ret, filesystem_option_build);
           ide_subprocess_launcher_push_argv (ret, build_dir_option);