travis: fix set_repositories_for(), needs sudo

.travis/build-pkgs

@@ -38,15 +38,15 @@ changed_abuilds() {
 # $1: the target repository; main, community, or testing
 set_repositories_for() {
 	local target_repo="$1"
-	local repos_file='/etc/apk/repositories'
+	local repos=''
 
-	printf '' > $repos_file
 	local repo; for repo in main community testing; do
-		printf '%s\n' "$MIRROR_URI/$repo" >> $repos_file
+		repos="$repos $MIRROR_URI/$repo"
 		[ "$repo" = "$target_repo" ] && break
 	done
 
-	apk update
+	sudo printf '%s\n' $repos > /etc/apk/repositories
+	sudo apk update
 }