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

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 <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>