[gnome-runtime-images] Make rust nightly image manual

[Jordan Petridis <jpetridis src gnome org>]

commit 0af5abc0f35a8846f80fed6ab48f703a3d84a651
Author: Jordan Petridis <jpetridis gnome org>
Date:   Thu Jun 25 09:53:39 2020 +0000

    Make rust nightly image manual
    
    Its failling since there isn't yet an sdk extension for fdo-sdk 20.08

 .gitlab-ci.yml | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 091b931..bbea534 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -171,6 +171,8 @@ gnome:3.36:
 rust:nightly:
   stage: 'rust_bundle'
   extends: ['.rust_template', '.vars_nightly']
+  rules:
+    - when: 'manual'
 
 rust:3.32:
   stage: 'rust_bundle'