cli: build-cfg and run-cfg are almost useless without nesting

[laurentsenta]

To be completed later

We use build args to pass parameters for the builder,
some / most build args are nested.

We would like to do something like:

testground run single \
    --plan="my-plan" --testcase="my-testcase" \
    --builder="docker:go" \
    --build-cfg "build_args.TEST_BROWSER_KIND=firefox"

• what happens if you pass in build_args={"TEST_BROWSER_KIND": "firefox"}?
• Can we merge these parameters so that "TEST_BROWSER_KIND" is not nested anymore?

cc @GlenDC (if you could add context)