Fix runner script env var construction

[elliotgunton]

• If user set their own env var we did not keep it

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.2%. Comparing base (25ac420) to head (8e21c9b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1289   +/-   ##
=====================================
  Coverage   86.2%   86.2%           
=====================================
  Files         60      60           
  Lines       4089    4091    +2     
  Branches     652     653    +1     
=====================================
+ Hits        3526    3530    +4     
+ Misses       392     391    -1     
+ Partials     171     170    -1     

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