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]

The committers listed above are authorized under a signed CLA.

• ✅ 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   +/-   ##
=======================================
  Coverage   59.28%   59.28%           
=======================================
  Files         126      126           
  Lines       17217    17217           
  Branches     3017     3017           
=======================================
  Hits        10207    10207           
  Misses       6325     6325           
  Partials      685      685           

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