[FIX] Use python's sys.version_info.minor to ensure correct library

To avoid finding multiple directories that have 'python' in the name
under /usr/local/lib/ on the GitHub Actions runners, use Python's
sys.version_info.minor to get the minor version of python3. As we know
that we're using Python 3, the minor version is the only piece of information
that needs to be determined.

.github/workflows/build.yml

@@ -282,8 +282,8 @@ jobs:
     - name: Verify Python bindings
       run: |
         export LD_LIBRARY_PATH="/usr/local/lib:${LD_LIBRARY_PATH}"
-        export PYTHON_VERSION_MINOR=$(_tmp=$(find /usr/local/lib/ -type d -iname "python*") && echo ${_tmp: -3}; unset _tmp)
-        export PYTHONPATH="/usr/local/lib/python${PYTHON_VERSION_MINOR}/site-packages:${PYTHONPATH}"
+        export PYTHON_VERSION_MINOR=$(python3 -c 'import sys; print(f"{sys.version_info.minor}")')
+        export PYTHONPATH="/usr/local/lib/python3.${PYTHON_VERSION_MINOR}/site-packages:${PYTHONPATH}"
         python3 -m pip list
         python3 -m pip show xrootd
         python3 -c 'import XRootD; print(XRootD)'