build: target amd64 — Google ships no arm64 Linux build-tools

The image pinned build-tools 36.0.0, whose aapt2/d8 are x86_64-only (Google
publishes no arm64 Linux build-tools). Built for and run on arm64, packaging
died with `aapt2: Syntax error: newline unexpected` (an x86_64 binary the
kernel can't exec).

Build the image for linux/amd64 on an amd64 runner, and point the template +
README at ubuntu-24.04. The APK output is architecture-neutral, so this costs
nothing downstream. Also execute aapt2 in the image sanity check so an arch
mismatch fails the image build, not every app build.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

templates/build.yml

@@ -24,7 +24,9 @@ env:
 
 jobs:
   build:
-    runs-on: ubuntu-24.04-arm64
+    # amd64: the toolchain image carries x86_64 Android build-tools (Google
+    # ships no arm64 Linux build-tools), so run it on an amd64 runner.
+    runs-on: ubuntu-24.04
     container:
       image: git.helu.ca/r/android:2026.06
       credentials: