fix: update build_rust_updater cbindgen inputs after flutter#350 API split (#140)

PR #138 (refactor: split shorebird C API consumption) switched
shell/common/shorebird/updater.cc to the new updater_engine.h header,
and the matching DEPS bump pulled the updater repo state where the
combined library/cbindgen.toml has been replaced with separate
library/cbindgen_dart.toml and library/cbindgen_engine.toml.

The build_rust_updater action's declared inputs were not updated to
match, so iOS engine builds on shorebird/dev fail with:

  ninja: error: '../../flutter/third_party/updater/library/cbindgen.toml',
  needed by 'gen/flutter/shell/common/shorebird/rust_updater_<...>.stamp',
  missing and no known rule to make it

This action only runs in shorebird-runtime engine builds, which the
periodic _build_engine workflow exercises but no per-PR CI job does,
so the regression went unnoticed.

Replace the missing input with the two split tomls.

Author: bdero

engine/src/flutter/shell/common/shorebird/BUILD.gn

@@ -86,7 +86,8 @@ if (shorebird_updater_supported) {
       "$shorebird_updater_dir/Cargo.lock",
       "$shorebird_updater_dir/library/Cargo.toml",
       "$shorebird_updater_dir/library/build.rs",
-      "$shorebird_updater_dir/library/cbindgen.toml",
+      "$shorebird_updater_dir/library/cbindgen_dart.toml",
+      "$shorebird_updater_dir/library/cbindgen_engine.toml",
       "$shorebird_updater_dir/library/.cargo/config.toml",
     ]
     inputs += shorebird_updater_rs_sources