Move environment variable declaration into env_vars list

[jhunkeler]

The sparse implementation of the BuildConfig class in testenv's groovy "accessor" does not include the runtime list member.

Applying environment variables...
Traceback (most recent call last):
  File "***/local/miniforge3/envs/TEMPORARY/bin/testenv", line 33, in <module>
    sys.exit(load_entry_point('testenv', 'console_scripts', 'testenv')())
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "***/testenv/testenv/cli/main.py", line 135, in main
    context = TestContext(pkg)
              ^^^^^^^^^^^^^^^^
  File "***/testenv/testenv/testcontext.py", line 108, in __init__
    self.apply_env_vars()
  File "***/testenv/testenv/testcontext.py", line 191, in apply_env_vars
    key, value = var.split('=')

The real error was:

$ groovy testenv/scripts/jfile_accessor.groovy REPO/JenkinsfileRT test_cmds
Caught: groovy.lang.MissingPropertyException: No such property: env for class: JenkinsfileRT
groovy.lang.MissingPropertyException: No such property: env for class: JenkinsfileRT
        at JenkinsfileRT.run(JenkinsfileRT:6)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 31.23%. Comparing base (92cfb36) to head (4f40946).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1814      +/-   ##
==========================================
- Coverage   33.18%   31.23%   -1.95%     
==========================================
  Files         126      159      +33     
  Lines       31164    35046    +3882     
  Branches     5778        0    -5778     
==========================================
+ Hits        10341    10946     +605     
- Misses      19654    24100    +4446     
+ Partials     1169        0    -1169     

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