feat: Add an option --pass-all-variants-as-env

[isuruf]

usually only used variables are sent to build scripts as env variables. this options sends all single element variables to build scripts. this is useful for conda-forge for variables like CROSSCOMPILING_EMULATOR

[isuruf]

@wolfv thoughts on this? If you agree, I can fix the CI and add some tests

[wolfv]

Hmm I don't think I can agree with this change as it makes it harder to recreate Conda-forge recipes.

What would you say if we add a top level "build env" key that will be used in all outputs?

Or actually, for a slightly hacky solution, if you use the variant keys in the global context then they should be available in all build environments. Or you can possibly use them with the global build.variants.use_keys: [foo, bar] and that will (probably) applied to all outputs (if not we should fix it!)

[isuruf]

No, requiring changes to hundreds of recipes to add CROSSCOMPILING_EMULATOR is too much work and error prone. Also, some variables like QEMU_LD_PREFIX are used by compiler activation variables and the recipe author doesn't even know about them. See https://github.com/conda-forge/ctng-compiler-activation-feedstock/blob/main/recipe/activate-gcc.sh#L201 and conda-forge/ctng-compiler-activation-feedstock#146