Respect $SATURNIN_CONF_PATH from environment

Turns out that leaving this to saturnin__conf_mkpath() is not so good
idea.

src/app.skel

@@ -20,7 +20,7 @@
 
 shellfu import saturnin
 
-SATURNIN_CONF_PATH="$(
+test -n "$SATURNIN_CONF_PATH" || SATURNIN_CONF_PATH="$(
     saturnin__conf_mkpath \
         "__SATURNIN_CONFIG_HOME__/ini.d" \
         "__SATURNIN_CONFIG_HOME__" \