attributes[:workdir] is empty when invoked via fpm-cookery
just rely on os value, don't define at all

--- ./lib/fpm/package/rpm.rb~	2015-08-10 23:07:09.000000000 +0300
+++ ./lib/fpm/package/rpm.rb	2015-08-21 17:11:59.163252070 +0300
@@ -423,7 +423,7 @@
       "--define", "_topdir #{build_path}",
       "--define", "_sourcedir #{build_path}",
       "--define", "_rpmdir #{build_path}/RPMS",
-      "--define", "_tmppath #{attributes[:workdir]}"
+#      "--define", "_tmppath #{attributes[:workdir]}"
     ]
 
     args += ["--sign"] if attributes[:rpm_sign?]