ci: provide a default for xcode generator

Provide a sane default for `CMAKE_GENERATOR` in the build script so that
it can be invoked without having to set that in the environment.

azure-pipelines/build.sh

@@ -11,6 +11,7 @@ SOURCE_DIR=${SOURCE_DIR:-$( cd "$( dirname "${BASH_SOURCE[0]}" )" && dirname $(
 BUILD_DIR=$(pwd)
 BUILD_PATH=${BUILD_PATH:=$PATH}
 CMAKE=$(which cmake)
+CMAKE_GENERATOR=${CMAKE_GENERATOR:-Unix Makefiles}
 
 indent() { sed "s/^/    /"; }