Local cache doesn't cleanup properly

Summary

Trying to build a project with no maximum size for the cache (hence defaults), I get:

[bschubert@me $MYELEMENT]$ cat eb0beee1-build.31021.log
[--:--:--] START   MYELEMENT.bst: Build
[--:--:--] LOG     MYELEMENT.bst: Build environment for element MYELEMENT.bst

    HOME: /tmp
    LC_ALL: C
    LOGNAME: tomjon
    PATH: /usr/bin:/bin:/usr/sbin:/sbin
    SHELL: /bin/sh
    SOURCE_DATE_EPOCH: '1320937200'
    TERM: dumb
    TZ: UTC
    USER: tomjon
    USERNAME: tomjon
[--:--:--] BUG     MYELEMENT.bst: Build

    An unhandled exception occured:

    Traceback (most recent call last):
      File "/home/bschubert/.local/lib/python3.6/site-packages/buildstream/_scheduler/jobs/job.py", line 417, in _child_action
        result = self.child_process()  # pylint: disable=assignment-from-no-return
      File "/home/bschubert/.local/lib/python3.6/site-packages/buildstream/_scheduler/jobs/elementjob.py", line 94, in child_process
        return self._action_cb(self._element)
      File "/home/bschubert/.local/lib/python3.6/site-packages/buildstream/_scheduler/queues/buildqueue.py", line 71, in process
        return element._assemble()
      File "/home/1/.local/lib/python3.6/site-packages/buildstream/element.py", line 1572, in _assemble
        self.__sandbox(rootdir, output_file, output_file, self.__sandbox_config) as sandbox:  # nopep8
      File "/opt/rh/rh-python36/root/usr/lib64/python3.6/contextlib.py", line 81, in __enter__
        return next(self.gen)
      File "/home/bschubert/.local/lib/python3.6/site-packages/buildstream/element.py", line 2252, in __sandbox
        allow_real_directory=not self.BST_VIRTUAL_DIRECTORY)
      File "/home/bschubert/.local/lib/python3.6/site-packages/buildstream/_platform/linux.py", line 69, in create_sandbox
        return self._create_bwrap_sandbox(*args, **kwargs)
      File "/home/bschubert/.local/lib/python3.6/site-packages/buildstream/_platform/linux.py", line 130, in _create_bwrap_sandbox
        return SandboxBwrap(*args, **kwargs)
      File "/home/bschubert/.local/lib/python3.6/site-packages/buildstream/sandbox/_sandboxbwrap.py", line 56, in __init__
        super().__init__(*args, **kwargs)
      File "/home/bschubert/.local/lib/python3.6/site-packages/buildstream/sandbox/sandbox.py", line 145, in __init__
        os.makedirs(directory_, exist_ok=True)
      File "/opt/rh/rh-python36/root/usr/lib64/python3.6/os.py", line 220, in makedirs
        mkdir(name, mode)
    OSError: [Errno 28] No space left on device: '/home/bschubert/.cache/buildstream/build/rmtfetickpublish-librmtfetickpublish-dev-f97w5sj6/root'

Steps to reproduce

• Create a small partition to use as your cache
• Build a project that will fill it up

What is the current bug behavior?

The cache cleaning doesn't kick in

What is the expected correct behavior?

Old entries in the cache should be removed

Relevant logs and/or screenshots

Possible fixes

Other relevant information

• BuildStream version affected: /milestone %BuildStream_v1.x

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