debug/buildinfo: update test for CL 369977

debug/buildinfo: update test for CL 369977

As a side effect of the changes in cmd/go/internal/work in CL 369977,
binaries built in GOPATH mode now include rudimentary build metadata
for at least the package path and compiler in use.

That seems like a strict improvement, but the test needs to be updated
to reflect the newly-available metadata.

Change-Id: I657c785e3e9992ed594c9524409f2d076f9eb376
Reviewed-on: https://go-review.googlesource.com/c/go/+/370234
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>