Use `opam config --root .. --switch .. exec sh -c 'jbuilder'`
Use `opam config --root .. --switch .. exec sh -c 'jbuilder'`

Unfortunately although the environment variables are set by
`opam config exec`, the call to `execvpe` does not use the new
`PATH`, so we have to filter the call to `jbuilder` through a
shell so it can search the `PATH`.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit