zshrc: simple-extract(): add fallbacks for .7z

[crpb]

% apt-file --filter-suites bookworm search bin/7z 7zip: /usr/bin/7zz p7zip: /usr/bin/7zr p7zip-full: /usr/bin/7z p7zip-full: /usr/bin/7za % apt-file --filter-suites trixie search bin/7z 7zip: /usr/bin/7z 7zip: /usr/bin/7za 7zip: /usr/bin/7zr 7zip-standalone: /usr/bin/7zz

Fixes #177

[ft]

Thanks! I'd probably have the assignment on a single line. Looks a little bit unusual like this, and I don't think we use this a lot elsewhere in the file. Rest looks good to me.