ci: use system python when available

.github/actions/bootstrap-poetry/action.yml

@@ -4,7 +4,7 @@ description: Configure the environment with the specified Python and Poetry vers
 inputs:
   python-version:
     description: Desired Python version expression
-    default: '3.12'
+    default: 'system'
   python-latest:
     description: Use an uncached Python if a newer match is available
     default: 'false'
@@ -28,13 +28,17 @@ runs:
   steps:
     - uses: actions/setup-python@v5
       id: setup-python
+      if: inputs.python-version != 'system'
       with:
         python-version: ${{ inputs.python-version }}
         check-latest: ${{ inputs.python-latest == 'true' }}
         allow-prereleases: ${{ inputs.python-prereleases == 'true' }}
         update-environment: false
 
-    - run: pipx install --python '${{ steps.setup-python.outputs.python-path }}' '${{ inputs.poetry-spec }}'
+    - run: >
+        pipx install \
+          ${{ inputs.python-version != 'system' && format('--python \"{0}\"', steps.setup-python.outputs.python-path) || '' }} \
+          '${{ inputs.poetry-spec }}'
       shell: bash
 
     # Enable handling long path names (+260 char) on the Windows platform