[enhancement]: Integration tests: change the cloud-init source in the actual image (not post-boot deb package installation)

# Enhancement

I know this is porbably an overkill (probably new security scan on the modified image per CI run) but it's worth reporting.

In https://github.com/canonical/cloud-init/pull/6860 , my new integration test was succeeding locally while failing in the CI. I am 99% sure because:
- the test involved changing the default user settings
- my local environment was lxd contianers so cloud-init source was used in place
- the github CI was using a debian package (so it is installed post-boot)

Even when I added a commit to cloud-init clean then reboot then do the test assertions, I got the same error

I guess this can be solved if we modify the cloud-init scripts in the image (there are tools to modify in-place files in static images) instead of building a deb package then installing it after the instance creation