CI: Fix OS check within GHA "Test" workflow

Author: amotl

.github/workflows/main.yml

@@ -59,7 +59,7 @@ jobs:
           bin/flake8
 
           test_program=bin/test
-          if [ ${{ matrix.os }} = "macos-latest" || ${{ matrix.os }} = "windows-latest" ]; then
+          if [[ ${{ matrix.os }} = "macos-latest" || ${{ matrix.os }} = "windows-latest" ]]; then
             test_program=bin/test-quick
           fi
           bin/coverage run $test_program -vv1