MNT: change circleCI install steps to fix erfa - #11081

OK, this was lame, and doesn't do the trick (even without the typo). However, ssh-ing in the job I indeed don't see the files in the _erfa directory:

-bash-3.2# ls _erfa/
core.py.templ	  erfa_generator.py  __init__.py  setup_package.py  ufunc.c.templ
erfa_additions.h  helpers.py	     __pycache__  tests

@mhvk - maybe the ssh into circleCI would provide a good enough start for you, too?

merging this, and as there are numerous issues with the backports due to branch divergences and dependency compatibility, I'll put those in a separate PR rather tha pushing directly on the branch.