Fix flakey macho test failures

[adfoster-r7]

The CI tests showed a ~6% chance of OSX payloads breaking:

bundle exec rspec ./spec/modules/payloads_spec.rb

Overriding user environment variable 'OPENSSL_CONF' to enable legacy functions.
Run options:
  include {:focus=>true}
  exclude {:acceptance=>true}

All examples were filtered out; ignoring {:focus=>true}

Randomized with seed 63326
Acceptance::DatastoreFormatting .......
modules/payloads ..............................................................................................................................................................................................................................................................................................................................................................................F.............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................

  1) modules/payloads osx/aarch64/meterpreter_reverse_https it should behave like payload cached size is consistent osx/aarch64/meterpreter_reverse_https can be instantiated and generated
     Failure/Error: raise "Invalid kSecCodeMagicEmbeddedSignature magic for macho" if s_magic != 0xfade0cc0

     RuntimeError:
       Invalid kSecCodeMagicEmbeddedSignature magic for macho
     Shared Example Group: "payload cached size is consistent" called from ./spec/modules/payloads_spec.rb:2536
     # ./lib/msf/core/payload/macho.rb:66:in `sign'
     # ./modules/payloads/singles/osx/aarch64/meterpreter_reverse_https.rb:41:in `generate'
     # ./lib/msf/core/payload.rb:309:in `generate_complete'
     # ./lib/msf/core/encoded_payload.rb:118:in `generate_raw'
     # ./lib/msf/core/encoded_payload.rb:74:in `generate'
     # ./lib/msf/core/encoded_payload.rb:24:in `create'
     # ./lib/msf/base/simple/payload.rb:52:in `generate_simple'
     # ./lib/msf/base/simple/payload.rb:139:in `generate_simple'
     # ./lib/msf/util/payload_cached_size.rb:174:in `compute_cached_size'

The TL:DR being:

Before: mettle embedded the config block with String#sub(pattern, replacement), and Ruby interprets backslash sequences (\0, \1, etc.) in the string replacement — so random config bytes containing \+char got expanded or dropped, changing the binary's size and shifting the Mach-O code signature out of place, which broke signing.

The fix uses the block form sub(pattern) { replacement }, which inserts the bytes verbatim without interpreting backreferences, so the binary keeps its exact size and the signature stays valid.

[smcintyre-r7]

The changes make sense and the tests pretty clearly illustrate what the issue is. Verified they pass with the changes and the tests fail without the changes to spec/metasploit_payloads/mettle/config_embedding_spec.rb.