cmd/audit: only flush formulary cache when needed

The cache here needs to be cleared when we are auditing multiple
os/arch combinations but not when we are running the audit only
for the current os/arch combination. This gives a 2x speed boost
to `brew audit --skip-style --except-version --tap=homebrew/core` locally.

In theory we could clear the cache less often by running all audits for
one os/arch combination at a time when auditing multiple os/arch combinations.
This would make the output a bit harder to follow and we don't seem to
run those checks on CI so I'm skipping that for now.

Library/Homebrew/dev-cmd/audit.rb

@@ -187,6 +187,8 @@ module Homebrew
     spdx_license_data = SPDX.license_data
     spdx_exception_data = SPDX.exception_data
 
+    clear_formulary_cache = [args.os, args.arch].any?
+
     formula_problems = audit_formulae.sort.each_with_object({}) do |f, problems|
       path = f.path
 
@@ -208,7 +210,7 @@ module Homebrew
         SimulateSystem.with os: os, arch: arch do
           odebug "Auditing Formula #{f} on os #{os} and arch #{arch}"
 
-          Formulary.clear_cache
+          Formulary.clear_cache if clear_formulary_cache
 
           audit_proc = proc { FormulaAuditor.new(Formulary.factory(path), **options).tap(&:audit) }