Fixes#745
This adds a script to create a release. Running it will create two
commits that would appear like this in `git log --oneline`:
67b5fff (master) release: reset released flag
0b808f3 (tag: v1.8.1) release: v1.8.1
100d2f3 docs: last change before release
It also re-creates the `version.nix` file, which is used by the flake
to determine the final version the disko CLI will print.
If `disko --version` is run from exactly the commit tagged `v1.8.1`, it
will print `1.8.1`. If it is run from any commit past that (like
master), it will print `1.8.1-67b5fff`, and if it is run from a local
folder with uncommitted changes, it will print `1.8.1-67b5fff-dirty`.
