Be explicit about CGO_ENABLED=1 for compare-cap build.

It looks like go1.18 is going to default to CGO_ENABLED=0, so force
CGO_ENABLED=1 when building this cap-libcap comparison program.

Fixes:

  https://bugzilla.kernel.org/show_bug.cgi?id=215603

Signed-off-by: Andrew G. Morgan <morgan@kernel.org>