circleci: switch to dedicated machine
circleci: switch to dedicated machine

This will provide a more stable system for performance sensitive
tests (like the stall detector test). The build still runs in
docker, since we need a controlled environment (and pre-installed
dependencies are nice).
Message-Id: <189d2f511f02e69c86ba356f334c52c7154a9724@scylladb.com>

scylladb/seastar,scylladb/seastar,avikivity/seastar,cloudius-systems/seastar,syuu1228/seastar,syuu1228/seastar,scylladb/seastar,avikivity/seastar,cloudius-systems/seastar,avikivity/seastar,cloudius-systems/seastar,syuu1228/seastar