test: ensure test-npm-install uses correct node

Currently it is possible that the shelled out instance of npm will use
the system copy of node. This PR changes the test to shim the build
directory into the path. This will ensure that npm will use the correct
version of node.

fixes: #6648

PR-URL: #6658
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

Author: Myles Borins

test/parallel/test-npm-install.js

@@ -33,7 +33,10 @@ const pkgPath = path.join(common.tmpDir, 'package.json');
 fs.writeFileSync(pkgPath, pkgContent);
 
 const proc = spawn(process.execPath, args, {
-  cwd: common.tmpDir
+  cwd: common.tmpDir,
+  env: {
+    PATH: path.dirname(process.execPath)
+  }
 });
 
 function handleExit(code, signalCode) {