windows: use unsafe.Add instead of pointer arithmetic on a uintptr

The existing uintptr arithmetic is arguably valid because the
environment block is not located within the Go heap
(see golang/go#58625).

However, unsafe.Add (added in Go 1.17) expresses the same logic with
fewer conversions, and in addition avoids triggering the unsafeptr
vet check.

For golang/go#41205.

Change-Id: Ifc509279a13fd707be570908ec779d8518b4f75b
Reviewed-on: https://go-review.googlesource.com/c/sys/+/492415
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>

windows/env_windows.go

@@ -37,14 +37,14 @@ func (token Token) Environ(inheritExisting bool) (env []string, err error) {
 		return nil, err
 	}
 	defer DestroyEnvironmentBlock(block)
-	blockp := uintptr(unsafe.Pointer(block))
+	blockp := unsafe.Pointer(block)
 	for {
-		entry := UTF16PtrToString((*uint16)(unsafe.Pointer(blockp)))
+		entry := UTF16PtrToString((*uint16)(blockp))
 		if len(entry) == 0 {
 			break
 		}
 		env = append(env, entry)
-		blockp += 2 * (uintptr(len(entry)) + 1)
+		blockp = unsafe.Add(blockp, 2*(len(entry)+1))
 	}
 	return env, nil
 }