install to-filesystem doesn't work when target /var is a separate mount

`install to-filesystem` fails with if the target filesystem includes an empty `/var` directory.

> ERROR Installing to filesystem: Verifying empty rootfs: Non-empty root filesystem; found "var"

With `--replace=wipe` the installation fails when the empty `/var` is an active mount point with

> ERROR Installing to filesystem: Device or resource busy (os error 16)

Start by reproducing `install to-filesystem` against a target with an empty `/var` directory on a separate mount. Compare normal installation with `--replace=wipe` and trace the filesystem verification and replacement paths. Done means both cases handle the active `/var` mount without reporting a non-empty root filesystem or a device-busy error.

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