Move env var config loading to be within the config package.

When the environment variable config file is loaded after config object
is initialized, we  end up loading the USE_RBE variable in the config
file after the code to start reproxy process is run. This causes a
problem when USE_RBE variable itself is set in the config files. To
avoid this, I've moved the config file loading code to be within the
config package, this makes the main.go file cleaner too.

Test:
Ran a build with USE_RBE:true set in the config json file. The build
fails without this change and succeeds with this change.

Bug: b/209486170
Change-Id: Iab3957b64f4b5456a861057d16ad318f4f78e0cb