Add --ignore-unknown-features to the CI.

[xStrom]

Having this flag enables the CI to continue working if the workspace gets a package that doesn't have e.g. the std feature.

This brings it in sync with the latest Linebender standard, see e.g. parley.

[DJMcNab]

LGTM

To clarify for future readers, this only applies to the features explicitly added.
There is a small risk that a package's std or libm feature is misnamed, but I think that's sufficiently unlikely as to be ignored.