use more readarray

The first part replaces a loop which is probably a performance
improvement, and the second part addresses a shellcheck complaint about
not declaring and assigning separately.

bin/nix-build-and-cache

@@ -25,12 +25,11 @@ just() {
 
     # Find all output paths of the installables and their build dependencies
     readarray -t derivations < <(nix path-info --derivation "$@")
-    cache=()
-    for derivation in "${derivations[@]}"; do
-        cache+=(
-            "$(nix-store --query --requisites --include-outputs "$derivation")"
-        )
-    done
+    readarray -t cache < <(
+        xargs \
+            nix-store --query --requisites --include-outputs \
+            <<< "${derivations[*]}"
+    )
 
     # Upload them to Attic
     #
@@ -62,12 +61,12 @@ ci() {
 
 # Build and cache all the package outputs
 packages() {
-    declare -a cache="($(
+    readarray -t cache < <(
         nix flake show --json 2> /dev/null |
             nix run --inputs-from "$toplevel" nixpkgs#jq -- \
             -r \
-            '.packages."x86_64-linux" | keys | map("'"$toplevel"'#" + .) | @sh'
-    ))"
+            '.packages."x86_64-linux" | keys | map("'"$toplevel"'#" + .) | .[]'
+    )
 
     just "${cache[@]}"
 }