fix(dotenv): prevent prototype pollution during parsing

[lucacasonato]

If you did

toString=FOO

that would overwrite Object.prototype.toString.

[marvinhagemeister]

LGTM

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.69%. Comparing base (ba0fc29) to head (f90a810).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6661      +/-   ##
==========================================
- Coverage   94.70%   94.69%   -0.01%     
==========================================
  Files         562      562              
  Lines       46646    46651       +5     
  Branches     6569     6570       +1     
==========================================
+ Hits        44174    44178       +4     
- Misses       2429     2430       +1     
  Partials       43       43              

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