Fix sphinx-build error arising when .github/workflows/readthedocs.yml is run

[evevkovacs]

This is the workflow to build the documentation on a weekly basis. The error is:

Run sphinx-build -T -E -b html -d docs/build/doctrees docs/source docs/build/html
/home/runner/work/_temp/3e3d9b21-c69f-479e-9561-807e262a6907.sh: line 1: sphinx-build: command not found
Error: Process completed with exit code 127.###

The workflow contains a line to mamba install sphinx, which should contain the sphinx-build command, so I'm not sure how to fix this error.