fix(ci): let the Android build scripts respect a caller-set ANDROID_HOME

build-android.sh hardcoded `export ANDROID_HOME="$HOME/Android/Sdk"`,
which discarded whatever the caller had set. In the builder image the SDK
is at /opt/android-sdk and the job exports exactly that, so the script
looked for an NDK under /root/Android/Sdk, found nothing, and the build
died a minute in with "Android SDK not found" -- the first automatic
`latest` APK build failed on this and nothing else.

Now a default rather than an override, matching what
test-player-conformance.sh already did. NDK_HOME likewise prefers an
explicitly pinned ANDROID_NDK_HOME over guessing with `ls | head -1`,
which is how CI pins an exact NDK revision.

A missing SDK now fails immediately and says which variable to set,
instead of letting `ls` print its own error and the real failure surface
a minute later inside the tauri CLI.

android-dev.sh had the same override and gets the same treatment.

scripts/android-dev.sh

@@ -7,7 +7,7 @@ echo "======================================"
 # Setup environment
 echo "Setting up environment..."
 source "$HOME/.cargo/env.fish" 2>/dev/null || source "$HOME/.cargo/env" || true
-export ANDROID_HOME="$HOME/Android/Sdk"
+export ANDROID_HOME="${ANDROID_HOME:-$HOME/Android/Sdk}"
 export NDK_HOME="$ANDROID_HOME/ndk/$(ls $ANDROID_HOME/ndk 2>/dev/null | head -1)"
 
 # Check prerequisites