go/build: set allTags even when short-circuiting x_GOOS_GOARCH.go

go/build: set allTags even when short-circuiting x_GOOS_GOARCH.go

Fixes #52053

Change-Id: I0e1f2737f97a4656913b34a731d8de2c77a15b4d
Reviewed-on: https://go-review.googlesource.com/c/go/+/396918
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>