packaging: fix docker-centos5 - use pythonexe and set to "python" as before

Fix 92a51a45d44c .

contrib/packaging/buildrpm

@@ -22,6 +22,7 b' while [ "$1" ]; do'
         shift
         PYTHONVER=2.7.16
         PYTHONMD5=f1a2ace631068444831d01485466ece0
+        PYTHONEXE=python
         ;;
     --rpmbuilddir )
         shift

contrib/packaging/mercurial.spec

@@ -2,6 +2,8 b''
 
 %define withpython %{nil}
 
+%global pythonexe python2
+
 %if "%{?withpython}"
 
 %global pythonver %{withpython}
@@ -15,7 +17,6 b''
 
 %else
 
-%global pythonexe python2
 %global pythonver %(%{pythonexe} -c 'import sys;print(".".join(map(str, sys.version_info[:2])))')
 
 %endif