libressl: postinstall code is mac-only (#72736)

Formula/libressl.rb

@@ -46,6 +46,7 @@ class Libressl < Formula
   end
 
   def post_install
+    on_macos do
       keychains = %w[
         /System/Library/Keychains/SystemRootCertificates.keychain
       ]
@@ -68,6 +69,7 @@ class Libressl < Formula
       rm_f %W[#{etc}/libressl/cert.pem #{etc}/libressl/cert.pem.default]
       (etc/"libressl/cert.pem").atomic_write(valid_certs.join("\n"))
     end
+  end
 
   def caveats
     <<~EOS