Build/Test Tools: Force run `grunt clean --dev` before `build`.

Seemingly as a side effect of [61438], the built code base contains a number of files and directories that have been moved or removed.

`grunt clean` was updated to be aware of the newly expected files in [62051], but the `clean` tasks required to ensure a pristine working state are not run prior to `build`.

This runs the `clean --dev` task is run immediately before `grunt build` runs as an attempt to clean up these files by ensuring a clean `src` directory.

Props jorbin.
See #64393.

git-svn-id: https://develop.svn.wordpress.org/trunk@62069 602fd350-edb4-49c9-b593-d223f7449a82