Add android ndk 26 to CI

[gavv]

Currently CI checks build with NDK 21 and NDK 25. We need to add NDK 26:

1. In dockerfiles repo, add ndk26 tag to toolchain-linux-android image. We need to add en entry to images.csv and ensure that everything builds well.

2. In main repo, update build.yml:

   • in android-linux job, modify matrix to check both ndk21 and ndk26 (for all ABIs)
   • in android-macos job, just switch from ndk 25 to ndk 26

Related docs:

• https://github.com/roc-streaming/roc-toolkit/blob/develop/docs/sphinx/development/continuous_integration.rst
• https://github.com/roc-streaming/roc-toolkit/blob/develop/docs/sphinx/portability/cross_compiling.rst

[emmhuber]

Hello, I will attempt to solve this!