Make template builder working on Debian

1. Use bash explicitly where required
2. Don't use bash-isms where not.

QubesOS/qubes-issues#1907

qubeize_image

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 
 if [ "$VERBOSE" -ge 2 -o "$DEBUG" == "1" ]; then
     set -x

templates.spec

@@ -33,11 +33,9 @@ Obsoletes:  %{name} > %{version}-%{release}
 Qubes template for %{template_name}
 
 %build
-pushd qubeized_images/%{template_name}
+cd qubeized_images/%{template_name}
 rm -f root.img.part.*
 tar --sparse --dereference -cf - root.img | split -d -b 1G - root.img.part.
-popd
-
 
 %install
 rm -rf $RPM_BUILD_ROOT