nix: make Rizin dev build self-contained

[b1llow]

Summary

• remove the github:b1llow/nix flake input and package Rizin locally with only the required Meson dependency fetch and injection logic
• pin the current Rizin dev commit and both fixed-output hashes in nix/rizin/checksum.json
• exclude the unused sigdb.wrap, which tracks a mutable branch and caused the dependency hash to drift
• expose packages.rizin, add an updater for the revision and both hashes, and build Rizin explicitly in CI
• include the Rizin VM context API adjustment from fix minor compilation error due to Rizin refactor #32

The updater can track the latest dev commit or accept an explicit full SHA:

./nix/rizin/update.sh
./nix/rizin/update.sh <commit>

It restores the previous checksum file if either Rizin or rz-tracetest fails to build and does not create a commit.

Validation

• nix build .#rizin --no-link --print-build-logs
• nix build .#rz-tracetest --no-link --print-build-logs
• ./nix/rizin/update.sh 38c3bed6c98ac52d234cf6c7eb9af7b860ce8c03
• nix flake check --all-systems --show-trace
• nix flake show --all-systems
• nix fmt -- --ci .
• ShellCheck on both new shell scripts

Related packaging work: b1llow/nix#4