CI: fix syntax error in workflow

Commit fa6299e introduced a syntax error in test-nix-install.yml. This
went unnoticed because GitHub's UI sucks pearl balls and doesn't show
you syntax errors at all.

Fix the syntax error.

Author: strager

.github/workflows/test-nix-install.yml

@@ -15,7 +15,6 @@ jobs:
     steps:
       - name: checkout
         uses: actions/checkout@v2
-      - name: checkout
 
       - name: install nix
         uses: cachix/install-nix-action@v12