simplify attic call

The comment isn't really needed since the reasoning applies to the
other instances of readarray and here-strings too, and the IFS bit is
unnecessary.

bin/nix-build-and-cache

@@ -15,11 +15,8 @@ just() {
         return
     fi
 
-    nix run --inputs-from . attic -- \
-        login \
-        "$ATTIC_SERVER" \
-        "$ATTIC_ENDPOINT" \
-        "$ATTIC_TOKEN"
+    nix run --inputs-from . attic#default -- \
+        login "$ATTIC_SERVER" "$ATTIC_ENDPOINT" "$ATTIC_TOKEN"
 
     # Find all output paths of the installables and their build dependencies
     readarray -t derivations < <(nix path-info --derivation "$@")
@@ -29,15 +26,13 @@ just() {
             <<< "${derivations[*]}"
     )
 
-    # Upload them to Attic
-    #
-    # Use `xargs` and a here-string because something would probably explode if
-    # several thousand arguments got passed to a command at once. Hopefully no
-    # store paths include a newline in them.
+    # Upload them to Attic. It seems to insist on newlines to separate the
+    # paths.
     (
         IFS=$'\n'
-        nix shell --inputs-from . attic -c xargs \
-            attic push "$ATTIC_SERVER:$ATTIC_CACHE" <<< "${cache[*]}"
+        nix run --inputs-from . attic#default -- \
+            push --stdin --no-closure "$ATTIC_SERVER:$ATTIC_CACHE" \
+            <<< "${cache[*]}"
     )
 }