do not apply patches in download mode

[invy]

When using download mode (--only-downloads) vcpkg still applies patches, which slows down the downloading.

[Step 3/9] Starting package 12/181: boost-type-traits:v142_x64-windows
[Step 3/9] Building package boost-type-traits[core]:v142_x64-windows...
[Step 3/9] -- Loading triplet configuration from: C:\teamcity\work\861de7c9201c3f79\vcpkg_data\triplets\v142_x64-windows.cmake
[Step 3/9] -- Installing port from location: C:\teamcity\work\861de7c9201c3f79\vcpkg_data\ports\boost-type-traits
[Step 3/9] -- Downloading https://github.com/boostorg/type_traits/archive/boost-1.70.0.tar.gz...
[Step 3/9] -- Extracting source C:/teamcity/work/861de7c9201c3f79/vcpkg_build/downloads/boostorg-type_traits-boost-1.70.0.tar.gz
[Step 3/9] -- Applying patch msvc-arm.patch
...