Clean up unused environment variable CIRQ_PRE_RELEASE_VERSION

[pavoljuhas]

As of #7187 setup.py files do not need CIRQ_PRE_RELEASE_VERSION
as an environment variable. Here we scrub it from the environment.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.50%. Comparing base (c517794) to head (2dd4002).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7614   +/-   ##
=======================================
  Coverage   97.50%   97.50%           
=======================================
  Files        1103     1103           
  Lines       99683    99683           
=======================================
  Hits        97198    97198           
  Misses       2485     2485           

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[mhucka]

LGTM