Fix %post and %postun generation in kmodtool

During zfs-kmod RPM build, $(uname -r) gets unintentionally evaluated on
the build host, once and for all. It should be evaluated during the
execution of the scriptlets on the installation host. Escaping the $
character avoids evaluating it during build.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Neal Gompa <ngompa@datto.com>
Signed-off-by: Samuel Verschelde <stormi-xcp@ylix.fr>
Closes #8866