bib: fix injecting default "build" arg into root for flags like --local

[mvo5]

[draft as this needs tests]

In PR#608 we dropped the container entrypoint and moved the logic to detect what command to run into the cobra parsing.

However it seems certain situations like:

bootc-image-builder  --type qcow2 --tls-verify=true  --local quay.io/centos-bootc/centos-bootc:stream10

are not detected correctly (thanka to Chunfu Wen for reporting!).

This commit detect this usage and injects the build arg then too.

Closes https://github.com/osbuild/bootc-image-builder/issues/614