feat: Implement additional `build="feat_*"` argument for pin functions

[wolfv]

Sometimes, users want to pin also on a certain build string.

In conda-build, this was possible by using something like:

- {{ pin_subpackage("foo", max_pin="x.x") }} feat_*

However, due to the nature of our evaluation, this string concatenation would not work. It's also less elegant, because the exact=true argument to the pins modifies the build string as well - so having one place where it's modified would be better!

The idea to fix this would be to add a new build="foo" argument to the pin functions, that is mututally exclusive with exact=true.

So that a user could write something like:

- ${{ pin_subpackage("foo", max_pin="x.x", build="feat_*") }}

It should also guard against two things:

• trailing non-whitespace characters after the pin_subpackage function (e.g. disallow ...) }} foo_*
• using exact=true and build="..." at the same time