build: upstream_url not used for setup

@aparcar Not sure how you want to handle this one...

In https://github.com/openwrt/asu/blob/main/asu/build.py#L211

where we run setup.sh, the value of environment["UPSTREAM_URL"] is not set, so the default FILE_HOST=mirror-03.infra... is used, even when the config settings are set to downloads.... On my local host, this bypasses the CDN for a bunch of files, most importantly the imagebuilder itself, and can cause timeouts on builds:

$ podman run -ti ghcr.io/openwrt/imagebuilder:mediatek-filogic-main

buildbot@984608bff4e5:~$ echo $FILE_HOST
mirror-03.infra.openwrt.org

buildbot@984608bff4e5:~$ . setup.sh
2026-04-16 13:07:31 URL:https://mirror-03.infra.openwrt.org/...

If I change the environment as below, works for me, but will that be ok on the production server?

asu/build.py line 163

-    environment: dict[str, str] = {}
+    environment: dict[str, str] = {
+        "UPSTREAM_URL": settings.upstream_url,
+    }

Start in asu/build.py around lines 163 and 211, then inspect how setup.sh reads UPSTREAM_URL and how settings.upstream_url is populated. Exercise the imagebuilder setup path with a configured upstream URL and confirm that setup uses it instead of the default mirror, including on the production configuration path.