release: Exclude container images from globs

We use a *.txz glob to get all of the "distributions" which comprise
the FreeBSD release, but we now (optionally) also build container
images which are .txz files. Grep those out from the distribution
lists.

A better long-term fix would probably be to generate an explicit list
of the .txz files we want rather than using an overbroad glob and
filtering out the files we *don't* want.

Fixes: d03c82c28da8 ("release: add optional OCI images")
MFC after: 1 minute
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