ci: devicetree_checks: Use specific version of runner image

This commit updates the "Devicetree script tests" workflow to use a
specific runner image version (ubuntu-20.04, macos-11, windows-2022)
instead of the latest version in order to prevent any potential
breakages due to the 'latest' version change by GitHub.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>