treewide: handle postPhases __structuredAttrs-agnostically

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

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

pkgs/development/interpreters/python/hooks/python-recompile-bytecode-hook.sh

@@ -20,5 +20,5 @@ pythonRecompileBytecodePhase () {
 }
 
 if [ -z "${dontUsePythonRecompileBytecode-}" ]; then
-    postPhases+=" pythonRecompileBytecodePhase"
+    appendToVar postPhases pythonRecompileBytecodePhase
 fi