Since #44522 it's possible to specify custom certificates for the Citrix
receiver. As it took me some time to create a proper setup Citrix can
behave fairly unexpected.
I mostly covered two aspects:
* Don't install Citrix with `nix run`: when `citrix.desktop` is linked
to $XDG_CONFIG_DIRS, it's possible to start a session directly from the
browser when loading `.ica` files which makes the usage *way* easier.
* It's possible to add custom certificates using the Citrix wrapper. A
new store path with the original derivation and the certificates will be
created and therefore no rebuild of the package is needed when adding
new certs.
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