CI: conditional Flutter install + concurrency + wget→curl for Mac runner

iOS workflows were broken on the Mac mini runner because Flutter
wasn't installed. Three fixes:

1) Conditional Flutter setup. Check 'flutter --version' first; only
   pull subosito/flutter-action@v2 if missing or wrong version. Self-
   hosted Mac runner persists SDKs between runs, so steady-state is
   zero-cost. First run on the runner triggers a full SDK download
   via subosito (with cache: true so subsequent fresh installs are
   still fast).

2) wget → curl across all iOS workflow steps. macOS doesn't ship
   wget by default; curl is always present and respects -fsSL +
   redirect properly.

3) Concurrency groups on all four workflows. A burst of commits no
   longer queues N copies of the same build — newer pushes cancel
   in-flight runs on the same branch. Groups:
     linux-<ref>, android-<ref>, ios-sim-<ref>, ios-testflight-<ref>

   This addresses the '6 runs from one merge' issue: each commit in
   the merge triggered all 3 push-listening workflows. Concurrency
   cancels the earlier 3 the moment the newer commit's runs start.

4) CocoaPods install is also conditional (was already 'if ! command -v
   pod' under the hood; just tightened the step name).

.github/workflows/build-linux.yml

@@ -11,6 +11,10 @@ on:
     branches: [dev, main]
   workflow_dispatch:
 
+concurrency:
+  group: linux-${{ github.ref }}
+  cancel-in-progress: true
+
 defaults:
   run:
     shell: bash