Caching buildtrees only works with BuildElement (cannot enter a shell after a failed build)

Summary

When I try to enter a shell on a build failure, I get an error instead.

Steps to reproduce

• default configuration of buildstream
• try to build an element that fails
• enter s at the prompt

What is the current bug behavior?

Build failure on element: vm/repo.bst

Choose one of the following options:
  (c)ontinue  - Continue queueing jobs as much as possible
  (q)uit      - Exit after all ongoing jobs complete
  (t)erminate - Terminate any ongoing jobs and exit
  (r)etry     - Retry this job
  (l)og       - View the full log file
  (s)hell     - Drop into a shell in the failed build sandbox

Pressing ^C will terminate jobs and exit


Choice: [continue]: s

Dropping into an interactive shell in the failed build sandbox

Error while attempting to create interactive shell: Buildtree is not cached locally or in available remotes

What is the expected correct behavior?

a shell into the failed build.

Other relevant information

• BuildStream version affected: 0591b0c8

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