Build: Ignore the vendor directory during the jsvalidate:build task.

Merges [43783] from the 5.0 branch to trunk.

Props netweb.
Fixes #45119.

Built from https://develop.svn.wordpress.org/trunk@44135


git-svn-id: https://core.svn.wordpress.org/trunk@43965 1a063a9b-81f0-0310-95a4-ce76da25c4cd