Add cppyy build isolation command in README

[aaronj0]

For newer OS's (Ubuntu 24, Fedora) users run into pip defaulting to the cppyy-cling pypi package instead of using the libcppyy and libcppyy-backend based on libInterOp. The addition of --no-build-isolation fixes this

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.69%. Comparing base (57d09bd) to head (fc6d2d5).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #328   +/-   ##
=======================================
  Coverage   73.69%   73.69%           
=======================================
  Files           8        8           
  Lines        3018     3018           
=======================================
  Hits         2224     2224           
  Misses        794      794