cmd/go: fix TestScript/vet_asm for OS without amd64

Add GOOS=linux to vet_asm script to avoid errors on OS not working on
amd64 architecture.

Change-Id: Ic3db43618008ae8e8bc08738c95c200dd4e916d7
Reviewed-on: https://go-review.googlesource.com/c/149577Reviewed-by: Alan Donovan <adonovan@google.com>
Run-TryBot: Alan Donovan <adonovan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/testdata/script/vet_asm.txt

 # Issue 27665. Verify that "go vet" analyzes non-Go files.
 
+env GOOS=linux
 env GOARCH=amd64
 ! go vet -asmdecl a
 stderr 'f: invalid MOVW of x'