Sometimes it's required to modify some parts of the Citrix build on
their own which is why `{pre,post}Install` hooks can be quite helpful.

Additionally some corporate clients use their own certificates that
aren't stored as trusted ones in the `cacert` package with all of the
trusted certs by Mozilla.

Now it's possible to add custom certs like this:

``` nix
with import <nixpkgs> { config.allowUnfree = true; };

let path = ../../Downloads/custom-corporate-cert.pem; in
citrix_receiver.override {
  extraCerts = [ path ];
}
```

All Citrix versions until 13.7 will be EOLed during the lifetime of
18.09: https://www.citrix.co.uk/support/product-lifecycle/milestones/receiver.html

citrix_receiver: allow custom certs and hooks for the installation