CI: add extra astropy pip index to pick up new dependency

[bsipocz]

This should fix the issue that we've been testing with a ~month old astropy dev version (as astropy-iers-data is a new dependency that hasn't been picked up with a newer dev wheel)

astropy/astropy#15146

This should fix the failures we see because of #2798

[codecov]

Codecov Report

Merging #2802 (358a4e9) into main (ba39fb2) will increase coverage by 0.01%.
Report is 15 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2802      +/-   ##
==========================================
+ Coverage   66.12%   66.13%   +0.01%     
==========================================
  Files         235      235              
  Lines       18080    18087       +7     
==========================================
+ Hits        11955    11962       +7     
  Misses       6125     6125              

Files Changed	Coverage Δ
astroquery/jplhorizons/__init__.py	100.00% <ø> (ø)
astroquery/jplhorizons/core.py	65.49% <ø> (ø)
astroquery/cds/__init__.py	50.00% <100.00%> (+7.14%)	⬆️
astroquery/exoplanet_orbit_database/__init__.py	100.00% <100.00%> (ø)
astroquery/ibe/__init__.py	100.00% <100.00%> (ø)
astroquery/irsa/__init__.py	100.00% <100.00%> (ø)
astroquery/irsa_dust/__init__.py	100.00% <100.00%> (ø)
astroquery/nasa_exoplanet_archive/__init__.py	100.00% <100.00%> (ø)
astroquery/ned/__init__.py	100.00% <100.00%> (ø)
astroquery/sha/__init__.py	100.00% <100.00%> (ø)

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