bst-external is marked for autoremoval from testing

[Debian testing autoremoval watch]

bst-external 0.18.0-1 is marked for autoremoval from testing on 2020-06-26

It (build-)depends on packages with these RC bugs:
959675: libgrpc++1: libgrpc++1: endless looping with default settings
961193: libgrpc++1: libgrpc++1 must be linked with libatomic on armel/mips/mipsel/powerpc
962197: buildstream: Event loop broken on Python 3.8
962205: buildstream: Missing dependency on bubblewrap