irmin: install binary as 'irmin', not 'nox-irmin'
The 46ba2cfe7 nox- prefix sweep moved (public_name irmin) to
(public_name nox-irmin), but the cram tests and homebrew.yml both
expect the binary to be reachable as 'irmin'. Restore that name and
keep 'nox-irmin' only as the opam package identifier.