opam: don't use the shell for no good reason

5 jobs for master in 7 minutes and 42 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #3981
fp-timing
build-coq8.6

00:01:49

passed #3980
fp-timing
build-coq8.6.1

00:02:50

passed #3979
fp-timing
build-coq8.7

00:04:33

passed #3978
fp-timing
build-coq8.7.dev

00:05:03

 
  Opam
passed #3982
opam

00:00:14