pkg/host: don't enable binfmt_misc on non-linux
pkg/host: don't enable binfmt_misc on non-linux

The csource options lack any relation to the tested OS,
while they do depend on the tested OS (most are linux-specific).
As the result we enable binfmt_misc, but its setup fails on anything
other than linux. As a work-around don't enable binfmt_misc on non-linux.

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