Hardcode the OPAMROOT

It shouldn't be necessary since it's the default, but might help with child
images that redefine HOME (apparently Github actions does that).
3 jobs for master in 17 seconds (queued for 15 seconds)
Status Job ID Name Coverage
  .Pre
passed #78020
make-targets

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  Build
failed build-images

 
  Publish
skipped #78022
publish-docker