Use '~' for $HOME in fish_plugins file rather than resolving path

[mattmc3]

The fish_plugins file isn't portable across machines for local plugins because it stores the path to $HOME, which could be different on a different machine. This PR fixes this.

Steps to reproduce with Fisher 4.4.4:

$ fisher install $__fish_config_dir/plugins/foo
fisher install version 4.4.4
Installing /Users/mattmc3/.config/fish/plugins/foo
           /Users/mattmc3/.config/fish/.fisher/functions/foo.fish
Updated 1 plugin/s
$ cat $__fish_config_dir/fish_plugins
jorgebucaran/fisher
/Users/mattmc3/.config/fish/plugins/foo

This PR for Fisher 4.4.5, which makes fish_plugins more portable across machines with alternative user names:

$ fisher install $__fish_config_dir/plugins/foo
fisher install version 4.4.5
Installing /Users/matt/.config/fish/plugins/foo
           /Users/matt/.config/fish/.fisher/functions/foo.fish
Updated 1 plugin/s
$ cat $__fish_config_dir/fish_plugins
jorgebucaran/fisher
~/.config/fish/plugins/foo

[jorgebucaran]

We missed this when we did the same for universal variables, so I really appreciate it. Thanks!

[jorgebucaran]

Just one more little thing! 🙏