Hide system pip when installing python (#5583)

python.sh

@@ -25,6 +25,10 @@ rsync -av --exclude '**/.git' $SOURCEDIR/ $BUILDDIR/
 # According to cmsdist, this is required to pick up our own version
 export LIBFFI_ROOT
 
+# If the python installer finds another pip, it won't install the new one
+export PATH=$(echo $PATH | awk -v RS=':' -v ORS=':' '!/python/ {print}' | sed 's/:$//')
+unset PYTHONUSERBASE
+
 # The only way to pass externals to Python
 LDFLAGS=
 CPPFLAGS=