Previously, `config.programs.zsh.dotDir` prepended strings with `$HOME`.
This caused issues like nix-community#5100, where `$HOME` is
inconsistently resolved in time for the evaluation of the option. The handling
of this variable is also inconsistent with how paths are handled elsewhere,
including within the same module, where `config.programs.zsh.history.path`
does not mutate the supplied string.
To preserve backwards compatibility, this change prepends
`config.home.homeDirectory` to relative paths, while assigning absolute paths
unchanged. Tests for both cases are added.
Signed-off-by: Austin Horstman <khaneliman12@gmail.com>
