Add support for using env variable for config file location

[jtduffy]

Resolves #2020

Now supports the use of the environment variable NEWRELIC_FILE to specify the location of the agent configuration file.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 71.42857% with 4 lines in your changes missing coverage. Please review.

Project coverage is 70.58%. Comparing base (ecc4ae5) to head (ff5a570). Report is 1 commits behind head on main.

Files	Patch %	Lines
...va/com/newrelic/agent/config/ConfigFileHelper.java	71.42%	3 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #2022   +/-   ##
=========================================
  Coverage     70.58%   70.58%           
- Complexity     9869     9875    +6     
=========================================
  Files           829      829           
  Lines         39915    39945   +30     
  Branches       6077     6078    +1     
=========================================
+ Hits          28173    28195   +22     
- Misses         9010     9020   +10     
+ Partials       2732     2730    -2     

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