community/opam: fix build

Fixes error found during 3.19 rebuild: https://build.alpinelinux.org/buildlogs/build-3-19-ppc64le/community/opam/opam-2.1.5-r1.log

Upstream has also removed the lines i have patched out here, but as part of a larger set of changes: https://github.com/ocaml/opam/commit/b3135db20