Set passthru.updateScript option to point to `update.sh`. This should
correctly now update cmctl automatically.
Signed-off-by: joshvanl <me@joshvanl.dev>
PR includes the version and git revision as ld build flags to ensure
correct version is output with the `cmctl version` command.
Adds an `update.sh` script which can be used to update the version of
cmctl. Will automatically fetch the latest semvar release of cmctl, and
update hashes accordingly in `default.nix`.
Signed-off-by: joshvanl <me@joshvanl.dev>