make sure npm is available in the env when building

There was the proper environment missing in the build() routine when npm wasn't installed globally. This led the build fail because `npm` could not be found. Calling getnvm to source the NVM env fixes it.

PKGBUILD

@@ -60,6 +60,8 @@ prepare() {
 }
 
 build() {
+    getnvm
+
     cd "${srcdir}/${pkgname%-git}"
 
     # We are not using the systems Electron as we need castlab's Electron.