mach: adopt uv and avoid system python (#34632)

This allows us to use `uv` for:
1. Installing a pinned Python version
2. Installing the dependency packages using `uv`'s pip compatible interface.
4. Bootstrapping `mach` without a Python installion on the host, using `uv
   run`

This change also introduces a new 'composite' GitHub action to setup
python in the different CI workflows. There is no support for externally
managed python installations and virtual environments. These could be
added in the future.

Fixes #34095, #34547

Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>

.github/workflows/windows.yml

@@ -114,13 +114,13 @@ jobs:
           choco install wixtoolset
           echo "C:\\Program Files (x86)\\WiX Toolset v3.11\\bin" | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append
       - uses: actions/setup-python@v5
-        if: ${{ ! fromJSON(needs.runner-select.outputs.is-self-hosted) }}
+      - name: Setup Python
+        uses: ./.github/actions/setup-python
         with:
-          python-version: "3.10"
+          skip-python-setup: ${{ fromJSON(needs.runner-select.outputs.is-self-hosted) }}
       - name: Bootstrap
         if: ${{ ! fromJSON(needs.runner-select.outputs.is-self-hosted) }}
         run: |
-          python -m pip install --upgrade pip
           python mach fetch
           python mach bootstrap-gstreamer
       # For some reason WiX isn't currently on the GitHub runner path. This is a