cmd/go: set TOOLEXEC_IMPORTPATH for -toolexec tools

This way, a -toolexec tool can tell precisely what package is being
built when it's run. This was very hard to do before, because the tool
had to piece together that information given the build action's
arguments or flags.

Since there wasn't a good set of tests for -toolexec, add one in the
form of a test script. It builds a simple set of packages with a variety
of build tools, to ensure that all the cases behave as expected.

Like other recent master changes, include the changelog item for this
user-facing change too.

Fixes #15677.

Change-Id: I0a5a1d9485840323ec138b2e64b7e7dd803fdf90
Reviewed-on: https://go-review.googlesource.com/c/go/+/263357


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Daniel Martí <mvdan@mvdan.cc>