* Refactor LoadFromEnv to use helpers
Avoid triggering the linter about the cyclomatic complexity of the LoadFromEnv
function in later changes by refactoring it to use setEnvString/setEnvBool
helpers.
No functional change, just collapse ~29 repetitive if-blocks into single-line
calls to two small helpers.
* Make Debian upstream repository configurable
Support overriding the Debian handler's upstream (e.g. Ubuntu archives)
via PROXY_UPSTREAM_DEBIAN or upstream.debian in the config file.
Tested with PROXY_UPSTREAM_DEBIAN=http://archive.ubuntu.com/ubuntu
to get Ubuntu Resolute packages.