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

5 jobs for master in 0 seconds (queued for 103 minutes and 6 seconds)
Status Job ID Name Coverage
  Build
failed #4046
fp-timing
build-coq8.6

failed #4045
fp-timing
build-coq8.6.1

failed #4044
fp-timing
build-coq8.7

failed #4043
fp-timing
build-coq8.7.dev

 
  Opam
skipped #4047
opam
 
Name Stage Failure
failed
build-coq8.6 Build There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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failed
build-coq8.6.1 Build There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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failed
build-coq8.7 Build There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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failed
build-coq8.7.dev Build There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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