internal/testenv: actually try to exec on ios and wasm

Due to a stray edit in CL 486275, the assignment to tryExecOk
in tryExec on ios would be immediately overwritten back to false.
This change fixes the stray edit.

Change-Id: I4f45fbf130dc912305e5f453b0d1a622ba199ad4
Reviewed-on: https://go-review.googlesource.com/c/go/+/488076
Reviewed-by: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>

Author: Bryan C. Mills

src/internal/testenv/exec.go

@@ -74,7 +74,7 @@ func init() {
 		cmd := exec.Command(exe, "-test.list=^$")
 		cmd.Env = origEnv
 		if err := cmd.Run(); err == nil {
-			tryExecOk = true
+			return true
 		}
 		return false
 	}