Github workflows: Fix platformio to use venv

Otherwise gets "This environment is externally managed"

.github/workflows/platformio_tests.yml

@@ -31,22 +31,25 @@ jobs:
 
       - name: Install and setup PlatformIO
         run: |
-          pip3 install -U platformio
-          export PATH=~/.local/bin:$PATH
+          python3 -m venv venv
+          venv/bin/pip3 install -U platformio
 
       - name: Build PlatformIO package
         run: |
+          source venv/bin/activate
           cd nanopb
           pio package pack
 
       - name: Example - Extract PlatformIO package to example dir
         run: |
+          source venv/bin/activate
           cp -R nanopb/examples/platformio example
           mkdir -p example/lib/nanopb
           tar -xzf nanopb/Nanopb-*.tar.gz -C example/lib/nanopb
 
       - name: Example - Build
         run: |
+          source venv/bin/activate
           cd example
           pio run
 
@@ -58,6 +61,7 @@ jobs:
 
       - name: Build in subdirectory with space characters
         run: |
+          source venv/bin/activate
           cp -R nanopb/examples/platformio "example with spaces"
           mkdir -p "example with spaces/lib/nanopb"
           tar -xzf nanopb/Nanopb-*.tar.gz -C "example with spaces/lib/nanopb"
@@ -66,6 +70,7 @@ jobs:
 
       - name: Build with default platformio.ini
         run: |
+          source venv/bin/activate
           mkdir -p test_default_pio_conf
           cd test_default_pio_conf
           pio project init