This adds the shell shebang to the wrapper script. Without this,
emacs and in particular agda2-mode (but probably other applications as
well) return a format error when trying to execute agda.
Instead it is provided to the user who can choose whether or not
to include it in the final derivati. Example of including would
be:
```nix
callPackage ... (self: { inherit (self.extras) extraThing; })
```
These extras are also available downstream without being built by
default. This is achieved with `passthru`.
- Only they are added to the optional build path (share/agda)
- Only they are are passed as an include dir (share/agda)
- Only they are propigatedBuildInputs
This is unused, future users can just use override `buildFlags`
and extend/replace as needed. `includeDirs` is provided for this
purpose.
We should add `dirOf self.everythingFile` rather than `.`, but
`dirOf` breaks on relative paths so that is not an option.
