Set default build process type in rezconfig or env var.

[predat]

This PR allows the user to define the name of the build process plugin to be used for build/release by defining it in the rezconfig or through an environment variable without having to specify it from the rez-build/rez-release command line tools.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 58.40%. Comparing base (36e0537) to head (3e5d178). Report is 37 commits behind head on main.

Files with missing lines	Patch %	Lines
src/rez/rezconfig.py	0.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1724      +/-   ##
==========================================
+ Coverage   58.29%   58.40%   +0.11%     
==========================================
  Files         126      126              
  Lines       17159    17165       +6     
  Branches     3505     3506       +1     
==========================================
+ Hits        10002    10026      +24     
+ Misses       6493     6474      -19     
- Partials      664      665       +1     

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[predat]

This PR is trivial; do you think you can merge it ?

[JeanChristopheMorinPerso]

Sorry for the delays on this. It looks good to me!