go.mod: update to 'go 1.17'
go.mod: update to 'go 1.17'

This enables module graph pruning for modules that depend on x/build,
and lazy loading within x/build proper.

It may also help somewhat with build timeouts for small Playground
programs that import from x/build, such as the one observed in
https://play.golang.org/p/Jj_dmqq6de0.

Change-Id: Ic963ebf8d6b3d1580e9c11012ce37a8d22e6ac11
Reviewed-on: https://go-review.googlesource.com/c/build/+/363494
Trust: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/build,golang/build,golang/build,golang/build