Right now we load `hm-session-vars.sh` from
`config.home.profileDirectory`, generally resulting in the following
code being generated in `zshenv`:
```
. "/etc/profiles/per-user/<username>/etc/profile.d/hm-session-vars.sh"
```
This is problematic in a few situations. For example, when entering a
`distrobox` environment I got the following error:
```
/home/<username>/.zshenv:.:2: no such file or directory: /etc/profiles/per-user/<username>/etc/profile.d/hm-session-vars.sh
```
Instead, let's point to the path directly to the `/nix/store` by using
`config.home.sessionVariablesPackage` instead. This is more robust and
it is more consistent with other things we source in ZSH.
9e0453a9b0