Use -Werror in GitHub Actions

[jendrikseipp]

I think it would be good to use the -Werror flag to turn compilation warnings into errors, but only for automated tests in GitHub Actions. (We don't want to break compilation for users simply because we compare an int to a size_t.) This can be achieved by running

export CXXFLAGS="-Werror" # Treat compilation warnings as errors.

in unittests.yml before building dlplan.

[drexlerd]

I added this line. I would have expected the compilation to fail because I never really cared about fixing those warnings but it didn't. Maybe it didn't work?

[jendrikseipp]

Probably you need to export the value in the same step as the cmake command.

[drexlerd]

done.