ci(shorebird): add shell steps for engine_stamp.json + aot-tools.dill

Both files are listed as shard artifacts in the new sharded build but
neither is actually produced — they were built outside the gn+ninja
graph in the legacy shell scripts (mac_build.sh / linux_build.sh) and
the sharded pipeline only knew about gn_ninja and rust step types.

shorebirdtech/_build_engine#209 adds a `shell` step type to
shard_runner. This PR uses it:

- mac-x64: new shell step runs `flutter/bin/et stamp` (writes
  out/engine_stamp.json) and copies the result into
  out/mac_release/engine_stamp.json so it ends up in the shard's
  tarball. The matching artifact src updates from "engine_stamp.json"
  to "mac_release/engine_stamp.json".

- linux/host: new shell step runs the host-built dart against
  flutter/third_party/dart/pkg/aot_tools to produce
  out/host_release/aot_tools/aot-tools.dill, matching what
  linux_build.sh:78-90 does. The existing artifact src
  ("host_release/aot_tools/aot-tools.dill") is already correct.

After this lands and an engine bump produces a SHA carrying it,
finalize will upload these two missing files and the sharded build's
prod artifact set should match the legacy build's exactly (modulo
iOS XCFramework signing).

Author: eseidel

shorebird/ci/shards/linux.json

@@ -91,6 +91,10 @@
         "gn_args": ["--no-prebuilt-dart-sdk"],
         "ninja_targets": ["flutter/build/archives:artifacts"],
         "out_dir": "host_debug"
+      },
+      {
+        "type": "shell",
+        "command": "mkdir -p out/host_release/aot_tools && out/host_release/dart-sdk/bin/dart pub get --offline --directory=flutter/third_party/dart/pkg/aot_tools && out/host_release/dart-sdk/bin/dart compile kernel flutter/third_party/dart/pkg/aot_tools/bin/aot_tools.dart -o out/host_release/aot_tools/aot-tools.dill"
       }
     ],
     "artifacts": [

shorebird/ci/shards/macos.json

@@ -157,12 +157,16 @@
         "gn_args": ["--runtime-mode=release", "--mac", "--mac-cpu=x64"],
         "ninja_targets": ["flutter/shell/platform/darwin/macos:zip_macos_flutter_framework", "flutter/lib/snapshot:generate_snapshot_bins", "flutter/build/archives:artifacts"],
         "out_dir": "mac_release"
+      },
+      {
+        "type": "shell",
+        "command": "flutter/bin/et stamp && cp out/engine_stamp.json out/mac_release/engine_stamp.json"
       }
     ],
     "compose_input": "macos-framework",
     "artifacts": [
       {"src": "host_release/dart-sdk", "dst": "flutter_infra_release/flutter/$engine/dart-sdk-darwin-x64.zip", "zip": true, "content_hash": true},
-      {"src": "engine_stamp.json", "dst": "flutter_infra_release/flutter/$engine/engine_stamp.json"}
+      {"src": "mac_release/engine_stamp.json", "dst": "flutter_infra_release/flutter/$engine/engine_stamp.json"}
     ]
   }
 }