Refactor extra1 to fertilizer_pump in HAL and main logic

- Renamed `extra1` method and related calls to `fertilizer_pump` for clarity and better domain alignment.
- Updated HAL implementation to control `extra2` GPIO for fertilizer pump operations.
- Added build script trigger to refresh `VERGEN_BUILD_TIMESTAMP` on each build.

Software/MainBoard/rust/build.rs

@@ -49,5 +49,8 @@ fn linker_be_nice() {
 
 fn main() {
     linker_be_nice();
+    // Non-existent path causes Cargo to always re-run this script,
+    // keeping VERGEN_BUILD_TIMESTAMP fresh on every build.
+    println!("cargo:rerun-if-changed=ALWAYS_REBUILD_SENTINEL");
     let _ = EmitBuilder::builder().all_git().all_build().emit();
 }