build: for default bootstrap, use Go 1.17 if present, falling back to Go 1.4

build: for default bootstrap, use Go 1.17 if present, falling back to Go 1.4

Preparation for #44505, but safe for Go 1.18.
Also fixes the default build on Macs, at least for
people who have a $HOME/go1.17 or have run

	go install golang.org/dl/go1.17@latest
	go1.17 download

Change-Id: I822f93e75498620fad87db2436376148c42f6bff
Reviewed-on: https://go-review.googlesource.com/c/go/+/369914
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>