Limit which slaves are used for dist snaps
Limit which slaves are used for dist snaps

Dist snaps run valgrind and our AMI bots are not using an up-to-date valgrind.

rust-lang-deprecated/rust-buildbot,edunham/rust-buildbot,edunham/rust-buildbot,rust-lang/rust-buildbot,rust-lang/rust-buildbot,rust-lang-deprecated/rust-buildbot,rust-lang-deprecated/rust-buildbot,rust-lang/rust-buildbot,edunham/rust-buildbot,aidanhs/rust-buildbot,aidanhs/rust-buildbot,aidanhs/rust-buildbot