cmd/dist: test goos, not gohostos, in tester.extLink

This error dates back to when the method was introduced in CL 6531.
It only matters for the rare case of building tests on one GOOS and
running them on another, and only makes a difference for the rare case
where one GOOS supports external linking and another does not.

Change-Id: I1a7abfb0a5bbec49ddbcd9c1a4f5c0ec43a8095c
Reviewed-on: https://go-review.googlesource.com/c/go/+/463991
Reviewed-by: Michael Knyszek <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>

src/cmd/dist/test.go

@@ -1087,7 +1087,7 @@ func (t *tester) out(v string) {
 }
 
 func (t *tester) extLink() bool {
-	pair := gohostos + "-" + goarch
+	pair := goos + "-" + goarch
 	switch pair {
 	case "aix-ppc64",
 		"android-arm", "android-arm64",