ci: fix github ssh deploy key (#499)

The one we use in the org keys is called CI_SSH, not GH_SSH.

Signed-off-by: Julia Vassiliki <julia.vassiliki@unsw.edu.au>

Author: midnightveil

.github/workflows/deploy.yml

@@ -245,4 +245,4 @@ jobs:
         manifest_repo: seL4/microkit-manifest
         manifest_branch: main
       env:
-        GH_SSH: ${{ secrets.GH_SSH }}
+        GH_SSH: ${{ secrets.CI_SSH }}