formulary: install tap if needed.

This matches what `cask_loader` does and is low risk given we no longer
allow tap pinning.

Fixes #7626

Library/Homebrew/formulary.rb

@@ -279,6 +279,8 @@ module Formulary
     end
 
     def load_file
+      tap.install unless tap.installed?
+
       super
     rescue MethodDeprecatedError => e
       e.issues_url = tap.issues_url || tap.to_s

Library/Homebrew/test/formulary_spec.rb

@@ -227,7 +227,8 @@ describe Formulary do
       end
     end
 
-    it "raises an error if the Formula is not available" do
+    it "raises an error if the Formula is not available after tapping" do
+      expect_any_instance_of(Tap).to receive(:install)
       expect {
         described_class.to_rack("a/b/#{formula_name}")
       }.to raise_error(TapFormulaUnavailableError)