mach: recreate .venv when .python-version changes (#34712)

The new images published in servo/ci-runners#12 should have `uv`
installed already and the initial build of servo triggered during the
base image construction will force the installation of the Python
version mentioned at the time of the image construction (3.12). When
.python-version changes, we can no longer use the .venv baked into the
image and must recreate the it to avoid activating the environment.

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

.github/workflows/linux.yml

@@ -129,14 +129,13 @@ jobs:
         if: ${{ runner.environment != 'self-hosted' && !inputs.upload }} # not needed on ubuntu 20.04 used for nightly
         run: echo "LIBCLANG_PATH=/usr/lib/llvm-14/lib" >> $GITHUB_ENV
       - name: Setup Python
+        if: ${{ runner.environment != 'self-hosted' }}
         uses: ./.github/actions/setup-python
-        with:
-          skip-python-setup: ${{ runner.environment == 'self-hosted' }}
       - name: Bootstrap dependencies
         if: ${{ runner.environment != 'self-hosted' }}
         run: |
           sudo apt update
-          python3 ./mach bootstrap --skip-lints
+          ./mach bootstrap --skip-lints
 
       # Always install crown, even on self-hosted runners, because it is tightly
       # coupled to the rustc version, and we may have the wrong version if the
@@ -146,10 +145,10 @@ jobs:
 
       - name: Build (${{ inputs.profile }})
         run: |
-          python3 ./mach build --use-crown --locked --${{ inputs.profile }} --features "layout_2013"
+          ./mach build --use-crown --locked --${{ inputs.profile }} --features "layout_2013"
           cp -r target/cargo-timings target/cargo-timings-linux
       - name: Smoketest
-        run: xvfb-run python3 ./mach smoketest --${{ inputs.profile  }}
+        run: xvfb-run ./mach smoketest --${{ inputs.profile  }}
       - name: Script tests
         run: ./mach test-scripts
       - name: Unit tests
@@ -158,7 +157,7 @@ jobs:
         with:
           timeout_minutes: 20
           max_attempts: 2 # https://github.com/servo/servo/issues/30683
-          command: python ./mach test-unit --${{ inputs.profile }}
+          command: ./mach test-unit --${{ inputs.profile }}
       - name: Archive build timing
         uses: actions/upload-artifact@v4
         with:
@@ -166,7 +165,7 @@ jobs:
           # Using a wildcard here ensures that the archive includes the path.
           path: target/cargo-timings-*
       - name: Build mach package
-        run: python3 ./mach package --${{ inputs.profile }}
+        run: ./mach package --${{ inputs.profile }}
       - name: Upload artifact for mach package
         uses: actions/upload-artifact@v4
         with:
@@ -175,7 +174,7 @@ jobs:
       - name: Upload nightly
         if: ${{ inputs.upload }}
         run: |
-          python3 ./mach upload-nightly linux \
+          ./mach upload-nightly linux \
             --secret-from-environment \
             --github-release-id ${{ inputs.github-release-id }}
         env: