Fix brew install --skip-post-install

Previous #15042 doesn't pass the flag all the way to
`FormulaInstaller.new`

Library/Homebrew/cmd/install.rb

@@ -250,6 +250,7 @@ module Homebrew
       quiet:                      args.quiet?,
       verbose:                    args.verbose?,
       dry_run:                    args.dry_run?,
+      skip_post_install:          args.skip_post_install?,
     )
 
     Upgrade.check_installed_dependents(

Library/Homebrew/install.rb

@@ -281,7 +281,8 @@ module Homebrew
       debug: false,
       quiet: false,
       verbose: false,
-      dry_run: false
+      dry_run: false,
+      skip_post_install: false
     )
       formula_installers = formulae_to_install.map do |formula|
         Migrator.migrate_if_needed(formula, force: force, dry_run: dry_run)
@@ -307,6 +308,7 @@ module Homebrew
           debug:                      debug,
           quiet:                      quiet,
           verbose:                    verbose,
+          skip_post_install:          skip_post_install,
         )
 
         begin