Avoid relying on project-level environment variables in CircleCI

We need to make some variables available on forks but some should better
not be available on forks. The security settings of CircleCI contexts on
organization-level allow that so it makes sense to avoid relying on
project-level variables.

Related ticket: https://progress.opensuse.org/issues/163187

.circleci/config.yml

@@ -331,7 +331,7 @@ workflows:
 
   # Nightly jobs are using the GitHub user "os-autoinst-bot" to create PRs (search for occurrences of `os-autoinst-bot`
   # within this repository). This user's access token is configured on https://github.com/settings/tokens as usual and set
-  # as variable `GITHUB_TOKEN` on https://app.circleci.com/settings/project/github/os-autoinst/openQA/environment-variables.
+  # as variable `GITHUB_TOKEN` on https://app.circleci.com/settings/organization/github/os-autoinst/contexts/200658e0-47ad-4806-bca3-eefa618e6611.
   nightly:
     triggers:
       - schedule:
@@ -347,6 +347,8 @@ workflows:
       - cache-npm
       - cache-fullstack:
           <<: *requires
-      - dependencies-pr
+      - dependencies-pr:
+          context: GitHub
       - build-docs-nightly:
+          context: GitHub
           <<: *requires_fullstack