* shell.nix: Allow to build on x86 darwin
I've tried to check out qdrant source code and build it on x86 mac
laptop, however, i found out a couple of issues with current
`shell.nix`.
- Couple of dependencies were missing.
- Default `CXXFLAGS` are not compatible with rocks-db (needs higher
deployment target).
- Mold setup did not work with clang out of the box.
This PR fixes all of the issues and allows to build `qdrant` from
within nix shell on Intel macOS. I don't have access to ARM Mac at the moment,
so I can not say for sure if this configuration will work there.
* Add comment to iconv
* Touch up
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Co-authored-by: xzfc <xzfcpw@gmail.com>
