tools/upgrade [nfc]: Push check_have_cocoapods down into each step

This will be convenient for adding a new step that doesn't require
CocoaPods.

The check is very fast, so unlike with check_pub_get_clean there's
no need to memoize the result.

tools/upgrade

@@ -111,6 +111,7 @@ check_pub_get_clean() {
 
 just_pod_update() {
     [ -z "${opt_pod}" ] && return;
+    check_have_cocoapods
 
     run_visibly pod update --project-directory=ios/
     run_visibly pod update --project-directory=macos/
@@ -123,6 +124,7 @@ upgrade_pod() {
         return
     fi
 
+    check_have_cocoapods
     check_no_uncommitted_or_untracked
     check_pub_get_clean
 
@@ -138,6 +140,7 @@ deps: Update CocoaPods pods (tools/upgrade pod)
 }
 
 upgrade_pub() {
+    check_have_cocoapods
     check_no_uncommitted_or_untracked
     check_pub_get_clean
 
@@ -170,6 +173,7 @@ deps: Upgrade packages within constraints (tools/upgrade pub)
 }
 
 upgrade_pub_major() {
+    check_have_cocoapods
     check_no_uncommitted_or_untracked
     check_pub_get_clean
 
@@ -221,8 +225,6 @@ EOF
     fi
 }
 
-check_have_cocoapods
-
 divider_line='================================================================'
 
 for step in "${opt_steps[@]}"; do