setup.sh: fatal: This word should yield a string, but it contains an array

remove implicit array comparison in case

pkgs/stdenv/generic/setup.sh

@@ -364,16 +364,13 @@ findInputs() {
     local var="${!varRef}"
     unset -v varVar varRef
 
-    # TODO(@Ericson2314): Restore using associative array once Darwin
-    # nix-shell doesn't use impure bash. This should replace the O(n)
-    # case with an O(1) hash map lookup, assuming bash is implemented
-    # well :D.
-    local varSlice="${var}[*]"
-    # ${..-} to hack around old bash empty array problem
-    case "${!varSlice-}" in
-        *" $pkg "*) return 0 ;;
-    esac
-    unset -v varSlice
+    # var is a reference to an array and can sometimes be undefined
+    # so checking the array with "${!var}[@]" does not work
+    # check if $pkgs is in the var ref array
+    # TODO(@Ericson2314): Restore using associative array
+    if [[ "${var}[*]" = *" $pkg "* ]]; then
+        return 0
+    fi
 
     eval "$var"'+=("$pkg")'