build-ndk-prebuilts.sh: disable bazel.

```
ERROR: /buildbot/src/android/master/out/soong/workspace/external/libcxx/BUILD.bazel:30:18: While resolving toolchains for target //external/libcxx:libc++: com.google.devtools.build.lib.packages.NoSuchTargetException: no such target '@soong_injection//product_config_platforms/products/ndk-eng:ndk-eng_android_riscv64': target 'ndk-eng_android_riscv64' not declared in package 'product_config_platforms/products/ndk-eng' defined by /buildbot/src/android/master/out/bazel/output/external/soong_injection/product_config_platforms/products/ndk-eng/BUILD (Tip: use `query "@soong_injection//product_config_platforms/products/ndk-eng:*"` to see all the targets in that package)
```

Bug: https://github.com/google/android-riscv64/issues/10
Bug: http://b/274509603
Bug: http://b/262192655
Test: build/soong/scripts/build-ndk-prebuilts.sh
Change-Id: I4b6633ab5aab3f01953b628d78ac1d79f2a4ee3c