CI: use PKG_SOURCE instead of PKG_NAME

CI: use PKG_SOURCE instead of PKG_NAME

The PKG_NAME is the installable name of a package while PKG_SOURCE is
the folder containig both `Makefile` and possibliy `test.sh`

This approach previously worked for packages where both NAME and SOURCE
are the same, e.g. `vim`, however fore more complex packages like
`mariadb` (SOURCE) the NAMES are partly
*mariadb-server-plugin-handlersocket*, which is no existing folder.

With this commit the `PKG_SOURCE` is used to find the `test.sh` script.

Signed-off-by: Paul Spooren <mail@aparcar.org>