tools/build/make.py: Avoid Python 3.7+ subprocess.run capture_output

This is just a convenient alias for setting stdout and stderr to PIPE,
so substitute it for that to be compatible with Python 3.6.

Fixes: 69cfdc81ea7b ("tools/build/make.py: Keep bootstrapped bmake binary up-to-date")

(cherry picked from commit b771d5705fc95445d4d81e054be664b7842156ff)