cmd/dist: stop trying to access runtime/internal/sys.a in debug mode

cmd/dist: stop trying to access runtime/internal/sys.a in debug mode

Now that we don't install the .a files, the installed
runtime/internal/sys.a no longer exists. Stop trying to access it.

Change-Id: Ic437e7ad95be6109d0b2fcfb81e840f5cb476be8
Reviewed-on: https://go-review.googlesource.com/c/go/+/451360
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>