In the past I've been able to use ports-mgmt/portdowngrade to downgrade ports to older versions. I haven't used this in a while, so I don't know if it still works since the move to git, but it might be worth a go.
Failing that, you could just patch the lang/nim port to use the previous version. The diff is here: <https://cgit.freebsd.org/ports/commit/?id=97fa651b911b8fdea521278404dbcd3378ea2415> Another option might be to use the quarterly package branch instead of latest as it might still have the last version, since I only updated the port a week or so ago.
