Load SSH config from env

Builds no longer depend on a preexisting config in ~/.ssh.

.gitlab-ci.yml

@@ -21,6 +21,8 @@ sync_mirror:
     - staging
   allow_failure: true
   script:
+    - mkdir ~/.ssh && echo "$GITHUB_HOST_KEY" > ~/.ssh/known_hosts
+    - eval $(ssh-agent -s) && ssh-add <(echo "$GITHUB_DEPLOY_KEY")
     - git clone --bare "$CI_BUILD_REPO" mirror.git
     - cd mirror.git
     - git update-ref -d refs/tags/production