Build firmware Github action fails with errors.

[Anomalocaridid]

The automatic build action seems to consistently fail with the following errors:

Unable to process command '::add-path::/home/runner/work/_temp/arm-none-eabi/bin' successfully.

The add-path command is disabled. Please upgrade to using Environment Files or opt into unsecure command execution by setting the ACTIONS_ALLOW_UNSECURE_COMMANDS environment variable to true. For more information see: https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

[Avamander]

Fixed by #579