fix syntax [skip circle] [skip azp]

Author: scott-huberty

.github/workflows/tests.yml

@@ -132,7 +132,7 @@ jobs:
       # Workaround macOS path behavior with login shells (which puts system Python first)
       - run: |  # zizmor: ignore[template-injection]
           if [[ $MNE_CI_KIND == "pip" ]];
-            prepend_path=$(dirname ${{ steps.setup-python.outputs.python-path }})
+            prepend_path="$(dirname ${{ steps.setup-python.outputs.python-path }})"
           elif [[ $MNE_CI_KIND == "pixi" ]]; then
             prepend_path="${{ github.workspace }}/tools/ci/macos-intel/.pixi/envs/default/bin"
           else