Revert "ci(shorebird): drop --offline from aot-tools pub get" (#148)

Restores the legacy `pub get --offline` form. The original failure
(missing very_good_analysis in shard pub cache) was a shard-environment
problem — sync.yaml ran tools/run_pub_get.py to populate the cache, but
shards' pub cache is per-runner ephemeral. Fixed properly in
shorebirdtech/_build_engine#209 (shard-build.yaml now also runs
run_pub_get.py post-sync), so shell steps can rely on --offline matching
legacy behavior. The JSON config shouldn't carry the env-prep detail.

(See FIXME in build_engine's shard-build.yaml: real fix is to pull
aot_tools deps via gclient like every other in-tree Dart package.)

Author: eseidel

shorebird/ci/shards/linux.json

@@ -94,7 +94,7 @@
       },
       {
         "type": "shell",
-        "command": "mkdir -p out/host_release/aot_tools && out/host_release/dart-sdk/bin/dart pub get --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"
+        "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": [