treewide: handle preInstallPhases __structuredAttrs-agnostically

Always specify the preInstallPhases attribute as a list instead of a
string.

Append elements to the preInstallPhases Bash variable using appendToVar
instead of string or Bash array concatenation.

pkgs/development/interpreters/python/hooks/python-runtime-deps-check-hook.sh

@@ -16,5 +16,5 @@ pythonRuntimeDepsCheckHook() {
 
 if [ -z "${dontCheckRuntimeDeps-}" ]; then
     echo "Using pythonRuntimeDepsCheckHook"
-    preInstallPhases+=" pythonRuntimeDepsCheckHook"
+    appendToVar preInstallPhases pythonRuntimeDepsCheckHook
 fi