test: restore no-gogcflags build shortcut, save time

With a clean cache on a laptop
before change
time go run run.go -- . fixedbugs
  real	2m10.195s
  user	3m16.547s
  sys	1m52.939s

Or, before, directly after make.bash (the actual use case we care about)
time go run run.go -- . fixedbugs
  real	2m8.704s
  user	3m12.327s
  sys	1m49.123s

after change
time go run run.go -- . fixedbugs
  real	1m38.915s
  user	2m38.389s
  sys	1m8.490s

Tests, fortunately, still seem to pass.

Latest version of this takes the slow route for cross-compilation, which includes wasm.

Change-Id: Iad19951612defa96c4e9830bce920c5e8733834a
Reviewed-on: https://go-review.googlesource.com/c/go/+/223083


Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>