Make clean work when a recipe has no recipe.sh.

Previously, one had to have a recipe.sh in all recipe directories in
order to run `make clean` successfully. This fixes that.

Signed-off-by: Wren Turkal <wt@penguintechs.org>

cook.sh

@@ -520,7 +520,9 @@ then
 
         cd "${COOKBOOK_RECIPE}"
 
-        source recipe.sh
+        if [ -e recipe.sh ]; then
+            source recipe.sh
+        fi
 
         ops=()
         for arg in "${@:2}"