os/exec: fix Win32 tests missing 'chcp'

'%SystemRoot%/System32/chcp.com' is a tool on Windows that
is used to change the active code page in the console.

'go test os/exec' can fail with:
"'chcp' is not recognized as an internal or external command"

The test uses a custom PATH variable but does not include
'%SystemRoot%/System32'. Always append that to PATH.

Updates #24709

Change-Id: I1ab83b326072e3f0086b391b836234bcfd8a1fb7
GitHub-Last-Rev: fb930529bb0673cdec921df5a2821c4b41de745e
GitHub-Pull-Request: golang/go#25088
Reviewed-on: https://go-review.googlesource.com/109361
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/exec/lp_windows_test.go

@@ -117,7 +117,7 @@ func createEnv(dir, PATH, PATHEXT string) []string {
 		dirs[i] = filepath.Join(dir, dirs[i])
 	}
 	path := strings.Join(dirs, ";")
-	env = updateEnv(env, "PATH", path)
+	env = updateEnv(env, "PATH", os.Getenv("SystemRoot") + "/System32;" + path)
 	return env
 }