conda: add mac os dependencies to conda environment

conda-environment.yml

@@ -1,10 +1,13 @@
+# NOTE: the # [platform] selectors have no effect in conda environment files
+# however, do work in conda-forge feedstocks
 name: cylc-dev
 channels:
   - conda-forge
 dependencies:
   - aiofiles >=0.7.0,<0.8.0
   - ansimarkup >=1.0.0
   - colorama >=0.4,<0.5
+  - coreutils >=8.0  # [osx]
   - cryptography
   - graphene >=2.1,<3
   - jinja2 ==2.11.0,<2.12
@@ -15,6 +18,7 @@ dependencies:
   - python
   - pyuv >=1.4.0,<1.5.0
   - pyzmq >=19.0.0,<19.1.0
+  - sed >=4.0  # [osx]
   - setuptools >=49
   - urwid >=2.0,<2.1
   #- rx >=1.6,<2 # TODO: https://github.com/conda-forge/cylc-feedstock/pull/3#issuecomment-660716268