transfer: Platform matcher should match multiple platforms

This allows arm64 to pull armhf images.
Before this change the transfer service would reject pulls for armhf on
an arm64 machine, or indeed any such platform variant mismatches.

I would argue that its a bit weird for the transfer service to reject a
pull at all since there are legitamate reasons to want to pull images
for other architectures, however that's a more philosophical change.

In the case where I ran into this, I have an arm64 machine running
an armhf containerd in an armhf container (for running some basic sanity
checks during packaging).
Tests started failing once `ctr` was moved to use the transfer service
by default.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

plugins/transfer/plugin.go

@@ -94,7 +94,7 @@ func init() {
 				}
 
 				var applier diff.Applier
-				target := platforms.OnlyStrict(p)
+				target := platforms.Only(p)
 				if uc.Differ != "" {
 					inst, err := ic.GetByID(plugins.DiffPlugin, uc.Differ)
 					if err != nil {