Force MSGPACK_DEFAULT_API_VERSION=1 on conan builds

Author: CanNuhlar

.github/workflows/ci-linux.yaml

@@ -25,7 +25,7 @@ jobs:
             compiler: gcc
             version: "10"
             conan: true
-            cmake-args: -DCMAKE_PREFIX_PATH=`pwd`/build -DCMAKE_MODULE_PATH=`pwd`/build
+            cmake-args: -DCMAKE_PREFIX_PATH=`pwd`/build -DCMAKE_MODULE_PATH=`pwd`/build -DMSGPACK_DEFAULT_API_VERSION=1
 
           - name: ubuntu-20.04-clang-10
             os: ubuntu-20.04
@@ -38,7 +38,7 @@ jobs:
             compiler: clang
             version: "10"
             conan: true
-            cmake-args: -DCMAKE_PREFIX_PATH=`pwd`/build -DCMAKE_MODULE_PATH=`pwd`/build -DAUTOBAHN_USE_LIBCXX=NO
+            cmake-args: -DCMAKE_PREFIX_PATH=`pwd`/build -DCMAKE_MODULE_PATH=`pwd`/build -DAUTOBAHN_USE_LIBCXX=NO -DMSGPACK_DEFAULT_API_VERSION=1
 
     steps:
       - uses: actions/checkout@v2