* discard unused variants before copying metadata

when variant matrix is large and mostly unused (as in conda-forge),
the length of input_variants may be several thousand
when only a few are actually used.

This causes `get_loop_vars` and `metadata.copy()` to become very expensive.

* try reducing with all used vars instead of loop vars

should reduce less

* perf: copy distributed variants list after subsetting

vastly reduces the number of copies computed for large variant matrices

* perf: pass used_vars subset to get_loop_vars

rather than computing all loop vars and then intersecting,
only consider relevant keys when computing loop vars

reduces get_used_loop_vars from O(n_vars * n_variants) to O(n_used_vars * n_variants)

* remove redundant deepcopy of config.variant

config.copy already copies this, no need to do it twice in metadata.copy

* add config.copy_variants method

to avoid calling pickle in too many places

* Update news/5392-variant-copy

* Add benchmark test for `render_recipe` (#5490)

---------

Co-authored-by: Matthew R. Becker <beckermr@users.noreply.github.com>
Co-authored-by: Bianca Henderson <beeankha@gmail.com>
Co-authored-by: Ken Odegard <kodegard@anaconda.com>

* Update .authors.yml

* Update news

* Updated authorship for 24.9.0

* Updated CHANGELOG for 24.9.0

---------

Co-authored-by: Ken Odegard <kodegard@anaconda.com>
Co-authored-by: Katherine Kinnaman <kkinnaman@anaconda.com>