Summary:
- Moved the export of `ZIG` and `ZIGFLAGS` from `tools/Unix.mk` to `tools/Export.mk`
- Simplified the `export` target in `tools/Unix.mk` by removing redundant `ZIG` and `ZIGFLAGS` variables
Impact:
- No functional changes - the `ZIG` and `ZIGFLAGS` variables are still exported correctly
- Improves code maintainability by centralizing environment variable exports in `Export.mk`
Signed-off-by: Huang Qi <huangqi3@xiaomi.com>