Julia Installer Crash Report (Better Unix Base directories specifications )

Hello, I am on an M1 Mac.
I encountered this error report:
```
name = 'Juliainstaller'
operating_system = 'unix:OSX'
crate_version = '1.6.4'
explanation = '''
Panic occurred in file 'src/operations.rs' at line 534
'''
cause = '''
called `Result::unwrap()` on an `Err` value: Failed to open juliaup config file.

Caused by:
No such file or directory (os error 2)'''
method = 'Panic'
backtrace = '''

0: 0x102dd664c - juliaup::command_config_modifypath::run_command_config_modifypath::h22055a10e45fa8e4
1: 0x102d1d794 - juliainstaller::main::hec60d2626a25bbd4
2: 0x102d0c340 - std::sys_common::backtrace::__rust_begin_short_backtrace::h38b7456c5d856e39
3: 0x102d2085c - _main'''
```

The installer configuration which I selected was the following:
```
✔ Do you want to install with these custom configuration choices? · Customize installation
✔ Enter the folder where you want to install Juliaup · /Users/user/.juliaup
✔ Do you want to add the Julia binaries to your PATH by manipulating various shell startup scripts? · no
✔ Do you want to add channel specific symlinks? · no
✔ Enter minutes between check for new version at julia startup, use 0 to disable · 1440
✔ Enter minutes between check for new version by a background task, use 0 to disable · 0
```
Most notably, I requested not to automatically modify the .zshrc file to add julia to the PATH (I rather do it by hand).
The culprit seems to be
```
pub fn remove_binfolder_from_path_in_shell_scripts() -> Result<()> {
let paths = find_shell_scripts_to_be_modified()?;

paths.into_iter().for_each(|p| {
remove_path_from_specific_file(p).unwrap();
});

Ok(())
}
```
I don't know rust enough, but since I requested no config file modifications, the variable `paths` should be empty, but we still try to iterate through it, without checks. Maybe rusts handles this on his own and the issue is further up?

Actually, I notice also that the function `find_shell_scripts_to_be_modified()` assumes that the `.zshrc` file is in the home folder. But i keep my `.zshrc` file in the `~/.config/zsh` folder. This assumption most likely caused the crash.

If that is the case a possible fix could be: Instead of defaulting to the home directory, use the `$ZDOTDIR` env variable (which I use to set the dot dir for zsh to the previously mentioned non-default location). Quoting the zsh man pages shipped with the OS:

```
Commands are then read from $ZDOTDIR/.zshenv. If the shell is a login shell,
commands are read from /etc/zprofile and then $ZDOTDIR/.zprofile.
Then, if the shell is interactive, commands are read from /etc/zshrc and then $ZDOTDIR/.zshrc.
Finally, if the shell is a login shell, /etc/zlogin and $ZDOTDIR/.zlogin are read.
[...]
If ZDOTDIR is unset, HOME is used instead.
```

P.S.: I found rather strange that the code went trough these routines even after electing to not have any startup file modified.

Start with src/operations.rs at the reported line and inspect remove_binfolder_from_path_in_shell_scripts() and find_shell_scripts_to_be_modified(). Reproduce the custom installation on macOS with shell-file modification disabled and a non-default ZDOTDIR, then verify the installer no longer panics and handles the selected shell configuration correctly.

Written by the indexing model from the issue text.