[Chapel Merge] Fix make docs if CHPL_HOME is not set

Branch: refs/heads/master
Revision: b11259f
Author: mppf
Log Message:

Merge pull request #16865 from mppf/make-docs-without-chpl-home-set

Fix make docs if CHPL_HOME is not set

I noticed that make docs did not work if CHPL_HOME was not set.
This PR fixes the issue.

Reviewed by @ben-albrecht - thanks!

  • [x] make docs succeeds without CHPL_HOME set
  • [x] full local testing

Modified Files:
M doc/util/chpl2rst.py

Compare: https://github.com/chapel-lang/chapel/compare/78878cac5771...b11259faa329