buildFHSUserEnv: Allow having custom /opt in the FHS environment

buildFHSUserEnv is meant primarily for running 3rd-party software
which is difficult to patch for NixOS. Such software is often built to
run from /opt. Currently, running such a software from FHS environment
is difficult for two reasons:

1. If the 3rd-party software is put into the Nix store via a simple
   derivation (with e.g. installPhase = "dpkg-deb -x $src $out"), the
   content of /opt directory of that derivation does not appear in the
   FHSEnv even if the derivation is specified in targetPkgs. This is
   why we change env.nix.

2. If using buildFHSUserEnvChroot and the host system has the /opt
   directory, it always gets bind-mounted to the FHSEnv even if some
   targetPkgs contain /opt (NB buildFHSUserEnvBubblewrap does not have
   this problem). If that directory is not accessible for non-root
   users (which is what docker's containerd does with /opt :-(), the
   user running the FHSEnv cannot use it.

   With the change in chrootenv.c, /opt is not bind-mounted to the
   container, but instead created as user-modifiable symlink to
   /host/opt (see the init attribute in
   build-fhs-userenv/default.nix). If needed, the user can remove this
   symlink and create an empty /opt directory which is under his/her
   control.

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