Use sh and not bash syntax

Author: tobiasdiez

.github/workflows/ci.yml

@@ -29,7 +29,7 @@ jobs:
     steps:
       - name: Prepare container
         run: |
-          if [[ "${{ matrix.container }}" == *"void-linux"* ]]; then
+          if [ "${{ matrix.container }}" = *"void-linux"* ]; then
             xbps-install -Syu xbps
             xbps-install -yu
             # node-based actions require libstdc++.so.6