The nightly builds for Android x86 (Marshmallow) are back for a while now and it seems they’re working as expected.
The bootable ISOs can be downloaded from this link for both 32bit and 64bit architectures. The build is made each night, freshly from the repository and unless the build fails, the ISO-s are updated.
It should boot either from USB or any other media. The page contains information about how to test it.