[gnome-runtime-images] ci: switch nightly base to fd.o-sdk 20.08

[Jordan Petridis <jpetridis src gnome org>]

commit f1a6325543744b20fa16b61bafa9c4cfc76a7b30
Author: Jordan Petridis <jpetridis gnome org>
Date:   Wed May 6 18:56:34 2020 +0300

    ci: switch nightly base to fd.o-sdk 20.08

 .gitlab-ci.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index bc07ae2..091b931 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -113,7 +113,7 @@ flat-manager-client:
 .vars_nightly:
   variables:
     BRANCH: master
-    FD_BRANCH: "19.08"
+    FD_BRANCH: "20.08"
   rules:
     - if: "$CI_MERGE_REQUEST_ID"
       when: 'always'