CI: Build filesystem .uf2 with dir2uf2.

[Gadgetoid]

I have created a test release with instructions for using the filesystem .uf2 here - https://github.com/pimoroni/enviro/releases/tag/v0.0.6-test

I've raised this as a draft PR because currently it just pulls my dir2uf2 GitHub repository and runs the utility from there. This all needs tidying up before it's ready for primetime.

TODO:

• [ ] - Publish dir2uf2 on pypi - though it needs a better name first!
• [x] - Move the diru2f manifest file (enviro.txt) into this repository
• [x] - Update this PR accordingly
• [x] - Test!
• [x] - Stretch Goal: Figure out how to join the two .uf2 files together into a final, one-shot firmware.

[Gadgetoid]

I'm pretty happy with this now. Although I mostly focused on the stretch goal rather than finalizing and publishing this tool, since that has the most immediate benefit to us.

Need to actually test a release!