cmd/go: scan $PATH to find version list in NewerToolchain

cmd/go: scan $PATH to find version list in NewerToolchain

NewerToolchain needs a list of candidate toolchains.
Currently it always consults the module version list, using the network.
When GOTOOLCHAIN=path, it should probably not do this,
both because =path implies we don't want to use the network
and because not every released version will be in $PATH.
Instead, scan $PATH to find the available versions.

For #57001.

Change-Id: I478612c88d1504704a3f53fcfc73d8d4eedae493
Reviewed-on: https://go-review.googlesource.com/c/go/+/499296
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Bypass: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>