os: avoid unnecessary usage of var

The `var` keyword is known to be problematic and is not needed here,
so better to use the `let` keyword for variable declarations.

PR-URL: nodejs#42563
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Akhil Marsonya <akhil.marsonya27@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/os.js

@@ -171,7 +171,7 @@ platform[SymbolToPrimitive] = () => process.platform;
  * @returns {string}
  */
 function tmpdir() {
-  var path;
+  let path;
   if (isWindows) {
     path = process.env.TEMP ||
            process.env.TMP ||
@@ -223,7 +223,7 @@ function getCIDR(address, netmask, family) {
   }
 
   const parts = StringPrototypeSplit(netmask, split);
-  for (var i = 0; i < parts.length; i++) {
+  for (let i = 0; i < parts.length; i++) {
     if (parts[i] !== '') {
       const binary = NumberParseInt(parts[i], range);
       const tmp = countBinaryOnes(binary);
@@ -261,7 +261,7 @@ function networkInterfaces() {
 
   if (data === undefined)
     return result;
-  for (var i = 0; i < data.length; i += 7) {
+  for (let i = 0; i < data.length; i += 7) {
     const name = data[i];
     const entry = {
       address: data[i + 1],