When running a debug build, the `sshca host sign` command will now check
the `SSHCA_CLI_DEBUG_TEST_MACHINE_ID` environment variable for the value
to use as the machine ID. This is useful during development and
testing, where the real machine ID is inaccessible or otherwise
unavailable.
The `SSHCA_CLI_DEBUG_TEST_MACHINE_ID` environment variable is *NOT* used
at all in release builds.