fix(nix): move build tools from buildInputs to nativeBuildInputs

[t4min0]

Summary

perl, clang, and pkg-config are build-time tools that need to be on $PATH during cargo build. In Nix, nativeBuildInputs are added to $PATH; buildInputs are not (they go to $NIX_LDFLAGS for runtime linkage).

With perl in buildInputs, the openssl-sys vendored build fails when the flake is consumed as an input from another flake:

cargo:warning=configuring OpenSSL build: Command 'perl' not found.

openssl stays in buildInputs (runtime linkage). clang, perl, pkg-config move to nativeBuildInputs (build tools, need $PATH).

Follow-up to #966. Fixes #894 for flake consumers.

Changes

• openssl: stays in buildInputs (runtime linkage dependency)
• clang, perl, pkg-config: moved to new nativeBuildInputs (build-time tools that must be on $PATH)
• No code changes, flake.nix only

Testing

• cargo clippy --workspace --all-targets -- -D warnings passes
• cargo test --workspace passes
• Live integration tested (if applicable)

Security

• No new unsafe code
• No secrets or API keys in diff
• User input validated at boundaries

[jaberjaber23]

Superseded by #1063, which does everything this PR does (move clang/perl/pkg-config to nativeBuildInputs) plus fixes the openfang-desktop GTK/tray runtime closure with wrapGAppsHook3 + libayatana-appindicator + the preFixup LD_LIBRARY_PATH wrapping required for a working NixOS tray. Closing in favor of #1063. Thanks @t4min0 for kicking this off — your change is effectively included.