Make import from derivation work again

Broken by f503e556ea
Fixes #12

Cause of the problem:
nix-instantiate uses the store in read-only mode by default, unlike
nix-build.
Import from derivation requires a read-write store, causing the
command to fail with a (bad) error message. This only happened when
the derivation wasn't already built. This did not occur often
because commands like arion up use nix-build and therefore typically
mask the problem on subsequent invocations.

To reproduce the problem, garbage collect, then arion ps.

src/arion

@@ -138,9 +138,11 @@ done
 
 do_eval() {
   echo 1>&2 "Evaluating configuration..."
+  # read-write-mode is required for import from derivation
   nix-instantiate \
     "$nix_dir/eval-docker-compose.nix" \
     --eval \
+    --read-write-mode \
     --json \
     --argstr uid "$UID" \
     --arg modules "$modules" \