Rerender broken for clang-compiler-activation-feedstock

[h-vetinari]

I'm opening this here more for visibility, because AFAICT it's a regression in conda-build: conda/conda-build#5443

However, since I wasn't able to determine the exact conditions that trigger the bug, it might be that other recipes are affected as well, hence this issue.

[jakirkham]

As it sounds like upstream this was due to undefined Jinja variables, it sounds like this is the same as issue: #2011