build: rework node resolution

This patch reworks and updates the node resolution logic for selecting a
node from a builder.

The new implementation reworks the logic to make use of containerd's
platforms.Matcher interface instead of manually associated strings, and
additionally provides a few behavioural changes over the original
implementation, namely platforms can be matched with non-strict
semantics. e.g. i386 builds can be scheduled on an amd64 node, arm/v6
builds can be scheduled on an arm/v7 node.

We also add a new collection of tests for tracking regressions and
making the intended behaviour clearer.

Signed-off-by: Justin Chadwell <[email protected]>