build: Check CI envvars before rev-parse

build: Check CI envvars before rev-parse

The CI pipeline does not do a full clone, so we cannot use `git
rev-parse` to get the short SHA checksum for the current commit.

Of course, since nothing's ever easy, we cannot use vcs_tag() with a
custom command, so we need to generate our out `demo_conf.h` header
straight from a script.