flatpak: Pin libportal dependency

Changes in libportal main branch break the CI and require dependency
changes which we shouldn't make on stable branch.

(cherry picked from commit 67927cb09b)

build-aux/flatpak/org.gnome.Nautilus.json

@@ -123,7 +123,8 @@
       "sources" : [
         {
           "type": "git",
-          "url": "https://github.com/flatpak/libportal.git"
+          "url": "https://github.com/flatpak/libportal.git",
+          "tag": "0.4"
         }
       ]
     },

build-aux/flatpak/org.gnome.Nautilus.yml

@@ -90,6 +90,7 @@ modules:
   sources:
   - type: git
     url: https://github.com/flatpak/libportal.git
+    tag: 0.4
 - buildsystem: meson
   builddir: true
   name: nautilus