Backport fix to unpin conda-build again

[h-vetinari]

The current meta.yaml does not have an upper bound for conda-build, but one was recently imposed by conda-forge/conda-forge-repodata-patches-feedstock#657 due to mamba-org/boa#392.

This keeps us from using the newest conda-build in CI (and thus some relevant fixes; c.f. conda-forge/conda-smithy#1844). Looking at mamba-org/boa#393, it shouldn't be a big issue to backport this PR?

CC @jaimergp @mbargull

[mbargull]

We'd also want mamba-org/boa#395 (or similar changes) for upcoming/future conda/conda-build version compat.

[jaimergp]

0.17.0 is coming (mamba-org/boa#396) so I think we won't need this.