execabs: don't override Go 1.19 error with our error

Go 1.19 incorporates the functionality of execabs directly.
If it has already reported an error, don't report our own error.

In particular Go 1.19 moved the error from lookPathErr to Err.
The code was already checking to not override lookPathErr.
With this change we also do not override Err.

Tested with Go 1.17 through Go 1.20.

Fixes golang/go#58606

Change-Id: I110127a3925f3800cc058d93e704604a59aa38f7
Reviewed-on: https://go-review.googlesource.com/c/sys/+/469735
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>

execabs/execabs_go118.go

@@ -7,6 +7,12 @@
 
 package execabs
 
+import "os/exec"
+
 func isGo119ErrDot(err error) bool {
 	return false
 }
+
+func isGo119ErrFieldSet(cmd *exec.Cmd) bool {
+	return false
+}