debian-build: fix checkinstall invocation (#15538)

Checkinstall had two problems:
1. when it was called without a version (e.g. with a derivation created
by fetchFromGitHub) it would use `src` as debian version, which caused
dpkg to fail
2. when dpkg failed, it would invoke the pager with the log, which hangs
the build

So now
1. the default version is the dummy `0.0.0`
2. the used pager is `cat`