add build argument for pin expressions

[wolfv]

This adds the ability to define build matchers for pin expressions.

For example:

- ${{ pin_subpackage("foo", lower_bound="x.x", build="cuda*") }}

Would apply a pin that will look something like foo >=1.2,<2.0a0 cuda*.

The build argument does not support modifying the actual build string.

The arguments exact=true and build="..." are mutually exclusive.

Fixes #1012