The provided get_cpm.cmake template will always try to download from github:

This causes builds to fail when run by a sandboxed builder. For example, I was hoping to package ada-url for nixpkgs, which uses your template verbatim, and had to patch it to not download if CPM_DOWNLOAD_LOCATION was set, eg:

-file(DOWNLOAD
+if(NOT (EXISTS ${CPM_DOWNLOAD_LOCATION}))
+  file(DOWNLOAD

Would you be open to making such a change upstream? Thanks!