The NDK‘s continuous builds can be accessed by anyone with a Google account at http://partner.android.com/build. Select “Build List” from the menu on the left and then choose a branch and target from the menus at the top of the page. Note that the “product” defaults to the macOS download, so if you’re using Linux or Windows you'll need to change that combo box too.
Disclaimer: These builds are not suitable for production use. This is just a continuous build. The amount of testing these builds have been put through is minimal. A successful build only means that our test suite built successfully on Linux and Darwin. Windows is not covered (the Windows build bots are actually Linux), and none of the tests have actually been run yet.
This is the master (development) branch of the NDK. Corresponds to https://android.googlesource.com/platform/manifest/+/master-ndk.
Release branches of the NDK. Note that the actual released versions are not tagged, but you can find the build number for the official release by examining the source.properties file in your NDK. The Pkg.Revision entry is in the format MAJOR.MINOR.BUILD (with beta or canary information being appended if appropriate).
win64_${ARCH} builds. Those shouldn't be in the list any more. You want win64.