Working directory of github actions in Build & Test badge

[Kwankyu Lee]

Hi,

If you look at the report of doctest failures there, you see for instance

sage -t --warn-long 68.6 --random-seed=... sage/structure/dynamic_class.py  

Then if you copy this into your sage root directory, it doesn't work since "src/" prefix is missing in the path of the source file. It seems the working directory is "SAGE_ROOT/src/" for the github actions. It would be convenient if it can be changed to "SAGE_ROOT".