Prior to 2.1.3:

dra@thor:~$ opam var ocaml:opamfile
/tmp/opam-18607-5b5b85/default/packages/ocaml/ocaml.5.0.0/opam

which was clearly an error. I couldn't find which fix is responsible in 2.1.3, but in 2.1.3 I get:

dra@thor:~$ opam var ocaml:opamfile
/home/dra/.opam/repo/default/packages/ocaml/ocaml.5.0.0/opam

but this is only slightly better, as the repository is in tar'd form, so the file doesn't actually exist.

I think it would be better if the final state of the variable pointed instead to /home/dra/.opam/5.0-beta/.opam-switch/packages/ocaml.5.0.0/opam (I can't remember if that's technically valid during the build?)