.github/workflows/ci.yaml: fix CI failure if xbps needs update

if the first xbps-install fails, then no repo index is kept, causing the
last xbps-install to fail with Package 'mdbook-linkcheck' not found in
repository pool.

.github/workflows/ci.yaml

@@ -10,7 +10,7 @@ jobs:
     steps:
       - name: Prepare container
         run: |
-          xbps-install -Syu || xbps-install -yu xbps
+          xbps-install -Syu || xbps-install -Syu xbps
           xbps-install -yu
           xbps-install -y mdbook-linkcheck vmdfmt git
       - uses: actions/checkout@v1