CI: rework build workflow to have split target and subtarget directly

CI: rework build workflow to have split target and subtarget directly

Instead of referring to a redundant job and ENV variables, rework build
workflow to accept and require split target and subtarget and use them
directly from inputs.

Rework each user and pass a JSON of tuple to matrix include with each
target/subtarget combination to test. Special notice this doesn't use
the github actions matrix combination feature but reference each
specific tuple of target and subtarget to test.

Just a cleanup no behaviour change intended.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>