Add env vars to sphinx config

[zklaus]

Readthedocs doesn't activate Conda environments and also doesn't use conda run. As a consequence, crucial environment variables are not set, preventing the successful building of the documentation.

This PR adds the environment variables in Sphinx' conf.py, providing a workaround for that problem. This might need to be updated in the future if either different variables are needed (e.g. Proj recently moved from PROJ_LIB to PROJ_DATA), or if Readthedocs addresses this, e.g. in readthedocs/readthedocs.org#5339.

Fixes #251

[codecov[bot]]

Codecov Report

Merging #258 (1d2985d) into main (8076f40) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #258   +/-   ##
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  Coverage   99.22%   99.22%           
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  Files          28       28           
  Lines        3102     3102           
=======================================
  Hits         3078     3078           
  Misses         24       24           

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[bjlittle]

Hey @zklaus, thanks for pushing this along 💯 🍻

I just approved your request for access to RTD as a maintainer for iris-esmf-regrd, so you're unblocked now 👍