Run unit tests and clang formatting as part of single-package CI build

[andrewmogan]

This PR partially addresses DUNE-DAQ/daq-release#544 by moving nightly code checks into the single-package CI builds. In addition to dbt-build --lint, this will also run dbt-build --unittest and dbt-clang-format.sh in the same container spun off the relevant image. In the interest of somewhat reducing code duplication, there's a dedicated step that generates a wrapper script which does the typical setup steps, e.g., cd $DBT_AREA_ROOT && source env.sh etc.

Running tests inside a container introduced a couple of quirks to do with permissions in Docker containers. To get around this, each docker run command now includes a --user $(id -u):$(id -g) to emulate a more typical non-root user experience. This in turn requires explicitly allowing others to write to the $GITHUB_WORKSPACE.

All artifacts are uploaded just as in the nightly code check workflow. Eventually, I want the CI dashboard workflow to pull these single-repo artifacts and structure the dashboard around individual repos rather than giant lists of tests. But for now, the nightly code checks and CI dashboard will proceed as normal until I implement this.

I primarily tested this using opmonlib since one of its unit tests attempts to write to a root area and expects an exception to be thrown.

Successful test in opmonlib: https://github.com/DUNE-DAQ/opmonlib/actions/runs/28816083789