Check licenses before building rather than after

[tjkirch]

If a license is incorrect and will be rejected, it's good to know that before
you've spent time building the project.

As an example, if the license in a Cargo.toml file is wrong and will be
rejected, but we check after the build, you have to rebuild that package even
though none of the source has changed.

---

Testing done:

I ran a build and check-licenses passed.

I then changed apiserver's Cargo.toml to have license = "Very-Bad-License" and saw it fail immediately, rather than building the whole OS, failing, and then requiring me to rebuild apiserver and the OS images.

I then built with cargo make -e BUILDSYS_ALLOW_FAILED_LICENSE_CHECK=true and saw the license error, but saw the build continue anyway.

[bcressey]

For local development, I've appreciated the ability to ignore cargo deny failures since we don't really model the expected version as part of our build process, and our integration seems to have changed in backwards-incompatible ways a few times now.

I like the change but I'd like to see a fix for #582 as a fast follow.

[tjkirch]

For local development, I've appreciated the ability to ignore cargo deny failures

@bcressey - I talked briefly with @iliana about this and we had the idea of setting an environment variable that would throw away the return value of the license check, for development purposes - what do you think?

[bcressey]

@bcressey - I talked briefly with @iliana about this and we had the idea of setting an environment variable that would throw away the return value of the license check, for development purposes - what do you think?

Would it be off by default, meaning "do not ignore the result"? That could work.

I'd prefer an approach like we have now with BUILDSYS_UPSTREAM_SOURCE_FALLBACK where we fail closed by default.

[tjkirch]

Would it be off by default, meaning "do not ignore the result"? That could work.

@bcressey Yeah, definitely failing safe. Something like BUILDSYS_ALLOW_FAILED_LICENSE_CHECK=true would be required, or it would fail the build fast. It'd probably include a message about that variable so someone doing local development would know how to skip the error temporarily.

[zmrow]

💼

[tjkirch]

⬆️ Rebase on develop.
⬇️ Add the ability to override via BUILDSYS_ALLOW_FAILED_LICENSE_CHECK=true as discussed in comments above.

Updated Testing Done above.