Expose /var/usrlocal if "--filesystem=host" is specified.

As /usr/local points to ../var/usrlocal on Silverblue,
/run/host/usr/local was previously a broken link inside the
sandbox. This patch checks if /var/usrlocal exists and bind-mounts it
to /run/host/var/usrlocal.

See bug #4010.
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