[CI] Use --upgrade to avoid changing directories for install of sdist

Install the sdist using

python3 -m pip --verbose install --upgrade ./dist/xrootd-*.tar.gz

to avoid having to change directories to move xrootd.egg-info off PYTHONPATH.

Note the use of --upgrade is only necessary on tags where there is properly
formatted SemVer version info in the sdist. A dev version like
'20220426.post7797547' doesn't need the --upgrade, but it is easier to just
always use it.

.github/workflows/build.yml

@@ -582,8 +582,7 @@ jobs:
       run: |
         cp packaging/wheel/* .
         ./publish.sh
-        cd ..  # Move xrootd.egg-info off PYTHONPATH
-        python3 -m pip --verbose install xrootd/dist/xrootd-*.tar.gz
+        python3 -m pip --verbose install --upgrade ./dist/xrootd-*.tar.gz
         python3 -m pip list
 
     - name: Verify Python bindings
@@ -629,8 +628,7 @@ jobs:
       run: |
         cp packaging/wheel/* .
         ./publish.sh
-        cd ..  # Move xrootd.egg-info off PYTHONPATH
-        python3 -m pip --verbose install xrootd/dist/xrootd-*.tar.gz
+        python3 -m pip --verbose install --upgrade ./dist/xrootd-*.tar.gz
         python3 -m pip list
 
     - name: Verify Python bindings
@@ -677,7 +675,7 @@ jobs:
       run: |
         cp packaging/wheel/* .
         ./publish.sh
-        python3 -m pip --verbose install ./dist/xrootd-*.tar.gz
+        python3 -m pip --verbose install --upgrade ./dist/xrootd-*.tar.gz
         python3 -m pip list
 
     - name: Show site-pacakges layout for XRootD modules