We shouldn't need it to persist now that we cache failed build dirs.

This change breaks the test
`tests/integration/shell.py::test_sysroot_workspace_visible`.
I can no longer see a use-case for this test.
AIUI, it tested that the failed build sysroot stored in the builddir has
the workspace's files in, despite the workspace being unmounted.
I believe this behaviour is made redundant by cached buildtrees.

This fixes part of #539

This includes changes in app.py:
* Interactive failure shell no longer uses the failed build sysroot,
  defaulting to the cached build tree.
* Always allow producing a failure shell.

Changes in element.py are:
* Errors caused by building don't store the failed build sysroot.
* When staging sources, will stage the element's cached build tree if it
  exists.

This fixes part of #539

This is related to #539