Include envdir command from builder image

The command was installed into the builder image (alongside the Python
library of the same name), but was never actually copied into the image
as intended.  I'd previously only used the library in the context of
pathogen builds but reached for the command recently only to find it
missing.

Dockerfile

@@ -198,6 +198,7 @@ COPY --from=builder /usr/lib/python3.6/site-packages/ /usr/lib/python3.6/site-pa
 COPY --from=builder \
     /usr/bin/augur \
     /usr/bin/aws \
+    /usr/bin/envdir \
     /usr/bin/nextstrain \
     /usr/bin/snakemake \
     /usr/bin/