Add comment explaining why make-fake-west-git is necessary?

[hab25]

I.e., right above

zmk-nix/nix/zephyr/fetcher.nix

Line 27 in 829adef

make-fake-west-git = writers.writePython3Bin "make-fake-west-git" {} ''

.

Even more importantly than helping maintenance of this project, after this is done, we could probably point the west maintainers to it so they could better consider adding a feature that obviates the need for this workaround. Then, by implementing that feature, this project would become more resistant to change because it would be relying on less fragile assumptions.

PS: Thanks a lot for this, it works flawlessly.