Memoize python3_sitearch for faster RPM spec file processing

This greatly speeds up the creation of source RPMs, among other
things.

samba.spec

@@ -169,6 +169,10 @@
 
 %global _systemd_extra "Environment=KRB5CCNAME=FILE:/run/samba/krb5cc_samba"
 
+# Make a copy of this variable to prevent repeated evaluation of the
+# embedded shell command.
+%global python3_sitearch %{python3_sitearch}
+
 Name:           samba
 Version:        %{samba_version}
 Release:        %{samba_release}%{?dist}