In Ros2, ament_vendor is mainly used like this:

ament_vendor(assimp_vendor
  SATISFIED ${assimp_FOUND}
  VCS_URL https://github.com/assimp/assimp.git
  VCS_VERSION v5.2.2
  CMAKE_ARGS
    -DASSIMP_BUILD_ASSIMP_TOOLS:BOOL=OFF
    -DASSIMP_BUILD_TESTS:BOOL=OFF
    -DASSIMP_BUILD_SAMPLES:BOOL=OFF
    "${ASSIMP_CMAKE_FLAGS}"
    "-DCMAKE_CXX_FLAGS=${ASSIMP_CXX_FLAGS}"
  PATCHES patches
)

and VCS_URL https://github.com/assimp/assimp.git is usually hardcoded.

When a mirror site of github is needed to build ros2, it's very difficult to set the mirror site URL unless the source code is modified.

I'm proposing an updated version of macro ament_vendor so that it can use env variable (if set) as a prioritized value of VCS_URL.

As a result, for all the xxxxx_vendor projects, it's not necessary to touch the source code of VCS_URL but to set an environmental value of assimp_vendor_VCS_URL in my case and use the mirror git site.

Any comment? I can create a PR for it.