ci: enable x86_64 for android master run

When https://github.com/lyft/envoy-mobile/blob/master/.github/workflows/artifacts.yml was introduced `--config=fat` was used. However, `x86_64` [did not work](https://github.com/lyft/envoy-mobile/runs/263105599).

```
ERROR: /Users/runner/runners/2.159.0/work/envoy-mobile/envoy-mobile/library/kotlin/src/io/envoyproxy/envoymobile/BUILD:6:1: Linking of rule '//library/kotlin/src/io/envoyproxy/envoymobile:envoy_jni' failed (Exit 1)
external/androidndk/ndk/toolchains/x86_64-4.9/prebuilt/darwin-x86_64/lib/gcc/x86_64-linux-android/4.9.x/../../../../x86_64-linux-android/bin/ld: error: symbol memcpy has undefined version GLIBC_2.2.5
clang: error: linker command failed with exit code 1 (use -v to see invocation)
Target //:android_dist failed to build
```

We need to investigate and fix.