shell.nix: supply `wrapFactor` helper to make standalone factor binary

shell.nix: supply `wrapFactor` helper to make standalone factor binary

This adds the shell function `wrapFactor`.  This function is intended to wrap
the result of calling `build.sh` in the shell environment so it can be executed
outside of the nix shell.

Example:

```
$ nix-shell
[nix-shell] $ ./build.sh bootstrap
...build factor vm and image...
[nix-shell] $ wrapFactor .
exit
$ ./factor
```

`wrapFactor` takes the path to the factor root dir as argument, and expects the
binary `factor` and the image file `factor.image` there and uses Nixpkgs'
`makeWrapper` to wrap the `factor` executable in-place with the correct
`LD_LIBRARY_PATH`.  Afterwards, the factor executable can be called outside of
the nix-shell environment.