Add zstandard to ambient setup

Used to be an indirect dependency of Augur, until xopen removed it
and we pivoted to the stdlib/backport in
<nextstrain/augur#2009>

Adding here to match the Nextstrain managed runtimes,
conda-base¹ and docker-base.²

¹ <https://github.com/nextstrain/conda-base/blob/ede4fa7518fc4e0ccbb6801909f1ecaeef880a7c/src/meta.yaml#L85>
² <nextstrain/docker-base#286>

Author: joverlee521

src/snippets/ambient-setup.rst

@@ -6,7 +6,7 @@
             --override-channels --strict-channel-priority \
             -c conda-forge -c bioconda --yes \
             augur auspice nextclade \
-            snakemake git epiweeks \
+            snakemake git epiweeks zstandard \
             ncbi-datasets-cli csvtk seqkit tsv-utils
 
 2. Activate the runtime: