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/desktops/gnustep/make/setup-hook.sh

···
     )
 }
 
-preInstallPhases+=" addGnustepInstallFlags"
+appendToVar preInstallPhases addGnustepInstallFlags
 
 addGNUstepEnvVars() {
     local filename

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

···
 
 if [ -z "${dontCheckRuntimeDeps-}" ]; then
     echo "Using pythonRuntimeDepsCheckHook"
-    preInstallPhases+=" pythonRuntimeDepsCheckHook"
+    appendToVar preInstallPhases pythonRuntimeDepsCheckHook
 fi

pkgs/development/libraries/glib/setup-hook.sh

···
 glibPreInstallPhase() {
   makeFlagsArray+=("gsettingsschemadir=${!outputLib}/share/gsettings-schemas/$name/glib-2.0/schemas/")
 }
-preInstallPhases+=" glibPreInstallPhase"
+appendToVar preInstallPhases glibPreInstallPhase
 
 glibPreFixupPhase() {
     # Move gschemas in case the install flag didn't help