multirhstest can't find Vc::Vector::index_type with Vc 1.3.3

Failing jobs:

• debian:10 gcc-8-noassert-17 build: Job #72799 failed for 9af998aa
• ubuntu:18.04 clang-6-17 build: Job #72805

The successful debian:10 clang-7-libcpp-17 build is probably due to Vc being disabled currently for clang 7, see docker/ci!29 (merged) and docker/ci!30 (merged).

Older images (debian:9, ubuntu:16.04) don't include Vc so are unaffected