Add HOMEBREW_NO_INSTALL_UPGRADE

Library/Homebrew/env_config.rb

@@ -257,6 +257,10 @@ module Homebrew
                      "`HOMEBREW_CLEANUP_PERIODIC_FULL_DAYS` days.",
         boolean:     true,
       },
+      HOMEBREW_NO_INSTALL_UPGRADE:            {
+        description: "If set, `brew install` will not automatically upgrade installed and out of date formulae",
+        boolean:     true,
+      },
       HOMEBREW_PRY:                           {
         description: "If set, use Pry for the `brew irb` command.",
         boolean:     true,

Library/Homebrew/formula_installer.rb

@@ -351,7 +351,12 @@ class FormulaInstaller
       message = <<~EOS
         #{formula.name} #{formula.linked_version} is already installed
       EOS
-      if only_deps?
+      if Homebrew::EnvConfig.no_install_upgrade? && formula.outdated? && !formula.head?
+        message += <<~EOS
+          To upgrade to #{formula.pkg_version}, run:
+            brew upgrade #{formula.full_name}
+        EOS
+      elsif only_deps?
         message = nil
       else
         # some other version is already installed *and* linked