Various improvments to zsh shell plugin

[mmdanggg2]

Fixes the shell not starting when using rez-env --norc
The originally used --rcs zsh argument does not specify an rc file as it does in bash, it simply enables reading rc files from $ZDOTDIR or $HOME. This was causing zsh to execute the rc file and exit.

Fixes broken shell prompt

[linux-foundation-easycla]

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• ✅ login: mmdanggg2 / name: James Horsley (7589243)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.28%. Comparing base (8f6f987) to head (7589243).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1865   +/-   ##
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  Coverage   59.28%   59.28%           
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  Files         126      126           
  Lines       17217    17217           
  Branches     3017     3017           
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  Hits        10207    10207           
  Misses       6325     6325           
  Partials      685      685           

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[JeanChristopheMorinPerso]

Hi @mmdanggg2, I think I would like to get this merged and released before the next release. But before merging it, I'd like to see why our tests didn't catch this problem before. See https://github.com/AcademySoftwareFoundation/rez/blob/main/src/rez/tests/test_shells.py#L255-L265. Because the shell code is quite 🍝 and complex, I'd really like to either fix the tests that didn't detect this problem or add a new test to make sure we can detect regressions in the future.

Let me know if you need help with these tests. They can be very picky at times.