tweak opam build-dep installation

3 jobs for master in 6 minutes and 50 seconds (queued for 6 seconds)
Status Name Job ID Coverage
  Build
passed build-coq8.6.1 #3730
fp-timing

00:03:50

passed build-coq8.7 #3729
fp-timing

00:06:37

 
  Opam
passed opam #3731

00:00:13