[gimp] flatpak: use libjxl 0.6 released version

[Daniel Novomeský <dnovomesky src gnome org>]

commit 9f8de1738a8c5e138c0b4c451cd64fdcd47b5b68
Author: Daniel Novomesky <dnovomesky gmail com>
Date:   Fri Oct 15 16:53:14 2021 +0200

    flatpak: use libjxl 0.6 released version

 build/flatpak/org.gimp.GIMP-nightly.json | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/build/flatpak/org.gimp.GIMP-nightly.json b/build/flatpak/org.gimp.GIMP-nightly.json
index 89e52b92fb..8d386559ea 100644
--- a/build/flatpak/org.gimp.GIMP-nightly.json
+++ b/build/flatpak/org.gimp.GIMP-nightly.json
@@ -631,8 +631,8 @@
                 {
                     "type": "git",
                     "url": "https://github.com/libjxl/libjxl.git";,
-                    /* Last commit on v0.6.x branch at time of writing. */
-                    "commit": "20cccbf646158cb128d68db20d12f93d1fb9b6a1"
+                    "tag": "v0.6",
+                    "commit": "25ef9145273eb5fe3db04e196327c77e404308d8"
                 }
             ]
         },