Support checkout for failure buildtrees

[nanonyme]

Sometimes it's not possible to enter a shell in buildtree. BuildStream 2 should support checking out buildtree so that it can be analyzed manually.

[nanonyme]

How I would suggest this to work is that when you try to artifact checkout failed build, any potential cached buildtree would be considered included and checked out.

[nanonyme]

Currently instead if you checkout failed build, you will silently get empty checkout

[nanonyme]

@juergbi turns out I already had feature request for this. So linking 83bfd04 to this so it will not get lost