Build/Test Tools: Copy vendor scripts earlier in the build.

Relocates the `copy-vendor-scripts` to run during the the `build:js` portion of the build script. This ensures the JavaScript files are in place before the `uglify:all` task is run.

Follow up to r61438

Props desrosj.
Fixes #65006. See #64393.


git-svn-id: https://develop.svn.wordpress.org/trunk@62189 602fd350-edb4-49c9-b593-d223f7449a82