Only change owners of the needed directories

Looks like when the whole /github directory owner is changed, then the
next job in the pipeline fails because of the wrong permissions.

entrypoint.sh

@@ -6,7 +6,7 @@ command=$3
 
 # '/github/workspace' is mounted as a volume and has owner set to root
 # set the owner to the 'build' user, so it can access package files
-sudo chown -R build /github
+sudo chown -R build /github/workspace /github/home
 
 # assumes that package files are in a subdirectory
 # of the same name as "pkgname", so this works well