meson: Use join_paths() instead '/'

6 jobs for windows-unittest in 36 minutes and 19 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed gst indent #66692

00:00:18

passed manifest #66691

00:00:20

 
  Build
passed build fedora x86_64 #66693

00:03:14

 
  Test
passed check fedora #66694

00:16:19

 
  Full Builds
passed build android arm64 api28 #66695

00:02:27

passed build cerbero fedora x86_64 #66696

00:13:38