fix: use shellHook to set aapt2 override via gradle.properties

AGP 8.7.3 reads android.aapt2FromMavenOverride via providers.gradleProperty()
which only reads gradle.properties files, not JVM system properties set via
GRADLE_OPTS -D. The shellHook rewrites the sentinel value on every nix develop
so the correct nix store path is always present before gradle runs.

flake.nix

@@ -54,9 +54,15 @@
 
           ANDROID_HOME = sdkRoot;
           ANDROID_SDK_ROOT = sdkRoot;
+
           # NixOS gotcha: AGP downloads a dynamically-linked aapt2 from Maven
-          # that can't run on NixOS. Point it at the SDK's own aapt2 instead.
+          # that can't run on NixOS. AGP reads the override from gradle.properties
-          GRADLE_OPTS = "-Dandroid.aapt2FromMavenOverride=${sdkRoot}/build-tools/${buildToolsVersion}/aapt2";
+          # as a project property (not a JVM system property), so GRADLE_OPTS -D
+          # flags don't work. The shellHook writes the correct nix store path
+          # directly into gradle.properties so providers.gradleProperty() finds it.
+          shellHook = ''
+            sed -i "s|android.aapt2FromMavenOverride=.*|android.aapt2FromMavenOverride=${sdkRoot}/build-tools/${buildToolsVersion}/aapt2|" gradle.properties
+          '';
         };
       }
     );

gradle.properties

@@ -1,2 +1,5 @@
 org.gradle.jvmargs=-Xmx2g
 android.useAndroidX=true
+# NixOS: AGP cannot run the Maven-downloaded aapt2 (dynamically linked generic Linux binary).
+# Value is rewritten by the nix shellHook on every `nix develop` — do not edit manually.
+android.aapt2FromMavenOverride=__NIX_SHELLHOOK_SETS_THIS__