ambient-setup: Add csvtk, seqkit, and tsv-utils

Prompted by nextstrain/measles#10 (comment)

These have become common commands used across Nextstrain pathogen repos¹
since they were added to the Docker runtime²

¹ https://github.com/search?q=org%3Anextstrain+AND+%28%22csvtk%22+OR+%22tsv-select%22+OR+%22seqkit%22%29&type=code
² nextstrain/docker-base@992c575

src/snippets/ambient-setup.rst

@@ -12,7 +12,7 @@
             -c conda-forge -c bioconda --yes \
             augur auspice nextclade \
             snakemake git epiweeks pangolin pangolearn \
-            ncbi-datasets-cli
+            ncbi-datasets-cli csvtk seqkit tsv-utils
 
 3. Set the runtime: