Tails Installer release doc: define variable that's needed even when doing purely packaging.

Let's not assume that the same person has just prepared a new upstream release.

wiki/src/contribute/release_process/tails-installer.mdwn

@@ -129,6 +129,7 @@ Update the Debian package for Tails
 Checkout the packaging branch, that would be `tails/wheezy` or `tails/jessie`,
 for example:
 
+	export PKG_NAME=tails-installer
 	export PACKAGING_BRANCH=tails/jessie
 	git checkout "$PACKAGING_BRANCH"