test/run: make GO_GCFLAGS mean same thing it does during make.bash

-gcflags=-flag means apply the flags only to the package named
on the command line (the main package, for these tests).

-gcflags=all=-flag means apply the flags to everything in the build,
including the standard library.

cmd/dist uses -gcflags=all=$GO_GCFLAGS, so test/run should do the same,
as the comment already explains, to avoid rebuilding the entire standard
library without the flags during test/run's builds.

We changed the scope of the flags without a pattern a few releases
ago and missed this one.

Change-Id: I039e60ca619d39e5b502261d4a73e1afc7e3f9fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/213827


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>