Support dumb terminals in bootstrap downloads

Dumb terminals can only interpret a limited number of control codes,
and rewriting the terminal buffer will make `./mach build` very talkative
on these terminals.

This can be tested by setting the environment variable TERM to "dumb"
as such:

	TERM=dumb ./mach build
Merge pull request #3405 from andreastt/ato/mach_dumb_term

Support dumb terminals in bootstrap downloads