Provide GNU coreutils in the runtime

They're provided in our other runtimes (almost by happenstance, as part
of the underlying OS image) and having them available in all runtimes
makes it much easier to write portable programs without having to deal
with GNU vs. BSD differences.

Note that typically GNU coreutils would already be available in the
Conda runtime on Linux (via the host system), but not the Conda runtime
on macOS (unless installed separately, e.g. via Homebrew).  So
explicitly including GNU coreutils here increases consistency,
isolation, and portability of the runtime.

Related-to: <nextstrain/ingest#41>

src/recipe.yaml

@@ -63,6 +63,7 @@ requirements:
     - bash
     - bzip2
     - csvtk
+    - coreutils
     - curl
     - epiweeks
     - git