This uses a step output from the test job to set the build matrix
for the board build jobs. The built boards depends on which files
were changed.
* Changes contained within ports/*/boards/ will build each board.
* Changes contained within ports/* will build all boards from the
port.
* All other file changes will build all boards.
All boards will be build for pushes in `adafruit/circuitpython` as
well.
A side-effect is that we no longer need to explicitly list the
boards to build. It will automatically update as new folders are
added.
Related to #4009
