make build-dep: prefer relative paths; they seem to work better on Windows

3 jobs for master in 6 minutes and 42 seconds
Status Name Job ID Coverage
  Build
passed build-coq8.6.1 #3752
fp-timing

00:03:36

passed build-coq8.7 #3751
fp-timing

00:06:28

 
  Opam
passed opam #3753

00:00:13