update tabs only if core formula installed

Closes Homebrew/homebrew#42745.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Library/Homebrew/cmd/update.rb

@@ -55,6 +55,7 @@ module Homebrew
       install_tap tap_user, tap_repo
       # update tap for each Tab
       tabs = dir.subdirs.each.map { |d| Tab.for_keg(Keg.new(d)) }
+      next if tabs.first.source["tap"] != "Homebrew/homebrew"
       tabs.each { |tab| tab.source["tap"] = "#{tap_user}/homebrew-#{tap_repo}" }
       tabs.each(&:write)
     end if load_tap_migrations

Library/Homebrew/migrator.rb

@@ -90,7 +90,7 @@ class Migrator
     # newname's tap is the same as tap to which oldname migrated, then we
     # can perform migrations and the taps for oldname and newname are the same.
     elsif TAP_MIGRATIONS && (rec = TAP_MIGRATIONS[formula.oldname]) \
-          && rec == formula.tap.sub("homebrew-", "")
+        && rec == formula.tap.sub("homebrew-", "") && old_tap == "Homebrew/homebrew"
       fix_tabs
       true
     elsif formula.tap