go/build: set PWD for go subcommands

go/build: set PWD for go subcommands

Since these commands already include an explicit Env field,
they will not be fixed automatically by proposal #50599.

Change-Id: Ia8157a71cf0cfe208bdc0da9aef54be3d26c795f
Reviewed-on: https://go-review.googlesource.com/c/go/+/391804
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Trust: Daniel Martí <mvdan@mvdan.cc>