Let users save their nix develop derivation in a profile by ignoring
this specific folder. It encourages the following workflow:
```
nix develop Toolchain/ --profile Toolchain/nix-profiles/dev
```
Which stops the dev enviornment being collected in the nix store. Later
devs can come back and do:
```
nix develop Toolchain/nix-profiles/dev
```
To continue where they left off, without having to download everything
from nixpkgs again.
Add a nix flake to `Toolchain/` that wraps the existing nix derivation
`Toolchain/serenity.nix`. This also comes with a lockfile making the nix
developer enviornment setup more reproducible.
We also update the documentation for "other builds" to mention the
flake.
Moves the nix script to setup the build environment from Documentation
into the Toolchain as a callable script. I also modified the script
to accept a "pkgs" argument to make it easy to override the nixpkgs
version from the command-line when calling the script.
I was building serenity on quite a fresh NixOS system and it turns
out `unzip` and `qemu` were missing from this nix expression to
compile and run serenity.
This removes the '$' character so that it is easier to copy commands
directly from the build instructions and then executing them without
first having to remove the '$' character.
