align prune with disk usage (du) command behaviour

[dysosmus]

The command buildctl du—-filter=... or the associated RPC supports a wider range of filters than the prune command.

> buildctl --addr unix:///var/lib/buildkit/buildkitd.sock du --filter='description~=".*go build.*"'
ID									RECLAIMABLE	SIZE	LAST ACCESSED
wt5gwgj5p8m7croouarjwp2vp*                                             	true       	1.49GB
k5ysmyy2eahyze4783hobeh3i*                                             	true       	1.49GB
kcae1sloyiibspvg0grlgukre*                                             	true       	1.48GB
> buildctl --addr unix:///var/lib/buildkit/buildkitd.sock prune --filter='description~=".*go build.*"'
Total:	0B

The expected behavior is that the prune command should match the same layers as the du command if they are reclaimable.

> buildctl --addr unix:///var/lib/buildkit/buildkitd.sock du --filter='description~=".*go build.*"'
ID									RECLAIMABLE	SIZE	LAST ACCESSED
wt5gwgj5p8m7croouarjwp2vp*                                             	true       	1.49GB
k5ysmyy2eahyze4783hobeh3i*                                             	true       	1.49GB
kcae1sloyiibspvg0grlgukre*                                             	true       	1.48GB
> buildctl --addr unix:///var/lib/buildkit/buildkitd.sock prune --filter='description~=".*go build.*"'
Total:	4.46GB

This would also allow finer GC control over the build cache when running buildkitd as a daemon.

[dysosmus]

@tonistiigi, thanks for the approval. The job test-os seems to have failed due to an unrelated / tmp error. Would you mind retriggering it?