[gnome-builder] build: fix python version in flatpak

[Christian Hergert <chergert src gnome org>]

commit 9460837e31349aa681ad3bd7f87aacf2a2bea2ea
Author: Christian Hergert <chergert redhat com>
Date:   Wed Oct 7 15:29:25 2020 -0700

    build: fix python version in flatpak
    
    The SDK uses python 3.8

 build-aux/flatpak/org.gnome.Builder.json | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/build-aux/flatpak/org.gnome.Builder.json b/build-aux/flatpak/org.gnome.Builder.json
index 3a7a03f45..ed277af57 100644
--- a/build-aux/flatpak/org.gnome.Builder.json
+++ b/build-aux/flatpak/org.gnome.Builder.json
@@ -639,7 +639,7 @@
                 "--buildtype=debugoptimized",
                 "-Dctags_path=/app/bin/ctags",
                 "-Dtcmalloc=true",
-                "-Dpython_libprefix=python3.7",
+                "-Dpython_libprefix=python3.8",
                 "-Dtracing=true",
                 "-Dhelp=true",
                 "-Dchannel=flatpak-nightly",