build: Use join_paths instead of concatenating the paths manually

3 jobs for fix-path-expension in 2 minutes and 22 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #29131
ubuntu-16.04

00:01:53

passed #29132
ubuntu-17.10

00:02:02

passed #29133
ubuntu-18.04

00:02:21