The workaround from your PR does not work for me, I still get a "wrong" ubuntu.sources file in /etc/apt/sources.list.d/ubuntu.sources and the same copied by curtin to /target/etc/apt/sources.list.d/ubuntu.sources.cutrin.orig
Both contain the default config with ubuntu.com as mirror and not my own config.
The workaround from your PR does not work for me, I still get a "wrong" ubuntu.sources file in /etc/apt/ sources. list.d/ ubuntu. sources and the same copied by curtin to /target/ etc/apt/ sources. list.d/ ubuntu. sources. cutrin. orig
Both contain the default config with ubuntu.com as mirror and not my own config.