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

Replay of CL 369914 after revert in CL 370138.
Only change is adding 'export GOROOT_BOOTSTRAP' in make.bash.

Change-Id: I8ced4e87a9dc0f05cc49095578b401ae6212ac85
Reviewed-on: https://go-review.googlesource.com/c/go/+/370274


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>