Bump actions/checkout from 2 to 3

Bumps [actions/checkout](https://github.com/actions/checkout) from 2 to 3.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](https://github.com/actions/checkout/compare/v2...v3)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/pio_build.yml

@@ -29,7 +29,7 @@ jobs:
     steps:
     - name: Show Build Flags
       run: echo $PLATFORMIO_BUILD_FLAGS
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - name: Cache pip
       uses: actions/cache@v2
       with: