CI: disable "update-environment" for "setup-python"

setup-python, by default, sets various cmake and pkg-config env
vars, so that packages using cmake can be built. Since this might
interfere with out package builds disable it.

We only care about the Python executable itself, so use the action
output to create the venv.

.github/workflows/build.yml

@@ -136,18 +136,23 @@ jobs:
         persist-credentials: false
 
     - uses: actions/setup-python@v5
+      id: python
       with:
         python-version: '3.13'
         cache: 'pip'
         cache-dependency-path: 'requirements.txt'
         # Note that ARM64 prior to Win11 requires x86, but this will install x64
         architecture: 'x64'
+        # Avoid it setting CMake/pkg-config variables
+        # https://github.com/actions/setup-python/blob/main/docs/advanced-usage.md#environment-variables
+        update-environment: false
 
     - name: Install deps
       env:
         PIP_DISABLE_PIP_VERSION_CHECK: 1
+        PYTHON_PATH: ${{ steps.python.outputs.python-path }}
       run: |
-        python -m venv .venv
+        & "$env:PYTHON_PATH" -m venv .venv
         .\.venv\Scripts\activate
         python -m pip install -r requirements.txt
         echo "$env:VIRTUAL_ENV\Scripts" | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append