execabs: let hasExec return false on wasip1

Wasm cannot execute processes. Follow CL 479622 and update hasExec to
match internal/testenv.HasExec.

Updates golang/go#58141

Change-Id: Ie44dc356ee589784c44906694fda387fb1448ad5
Reviewed-on: https://go-review.googlesource.com/c/sys/+/485655
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

execabs/execabs_test.go

@@ -21,7 +21,7 @@ import (
 // Copied from internal/testenv.HasExec
 func hasExec() bool {
 	switch runtime.GOOS {
-	case "js", "ios":
+	case "wasip1", "js", "ios":
 		return false
 	}
 	return true