In element, platform asks for host os and architecture to default to when
SandboxConfig is initialised.
This changes element cache keys so those have been updated in the tests.

Also update tests to be consistent with this

These are contained within the platform field.

Same format as OptionArch tests, but with OS's.

  o _platform/linux.py: Add linux32 flag to send to sandbox bwrap when the
    build arch is x86-32 and the machines arch is x86-64 or similarly with
    aarch32 and aarch64.
  o sandbox/_sandboxbwrap.py: Use flag passed to start bwrap command with
    linux32 if set.

Execution environment reqs

Closes #775

See merge request !969