workspace: introduce bazel downloader config and replace bazeldnf mirror

This rather small change does rework the way we cache our dependencies
by instructing bazel to fetch everything through a custom proxy. See
//build/mirror_proxy:README.me for more infos.

Closes monogon-dev/monogon#178

Change-Id: Ic671fc8233a1cbf37427bbc96339ea8108310e21
Reviewed-on: https://review.monogon.dev/c/monogon/+/3686
Tested-by: Jenkins CI
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
diff --git a/third_party/sandboxroot/repo.yaml b/third_party/sandboxroot/repo.yaml
index e0dc51d..86141e9 100644
--- a/third_party/sandboxroot/repo.yaml
+++ b/third_party/sandboxroot/repo.yaml
@@ -3,19 +3,11 @@
   metalink: https://mirrors.fedoraproject.org/metalink?repo=fedora-40&arch=x86_64
   name: 40-x86_64-primary-repo
   mirrors:
-    - https://ftp.fau.de/fedora/linux/releases/40/Everything/x86_64/os
-    - https://ftp.halifax.rwth-aachen.de/fedora/linux/releases/40/Everything/x86_64/os
-    - https://mirror.23m.com/fedora/linux/releases/40/Everything/x86_64/os
-    - https://ftp.plusline.net/fedora/linux/releases/40/Everything/x86_64/os
-    - https://storage.googleapis.com/monogon-infra-public/mirror/fedora/linux/releases/40/Everything/x86_64/os
+    - https://dl.fedoraproject.org/pub/fedora/linux/releases/40/Everything/x86_64/os
   gpgkey: https://storage.googleapis.com/monogon-infra-public/RPM-GPG-KEY-fedora-40-primary
 - arch: x86_64
   metalink: https://mirrors.fedoraproject.org/metalink?repo=updates-released-f40&arch=x86_64
   name: 40-x86_64-update-repo
   gpgkey: https://storage.googleapis.com/monogon-infra-public/RPM-GPG-KEY-fedora-40-primary
   mirrors:
-    - https://ftp.fau.de/fedora/linux/updates/40/Everything/x86_64
-    - https://ftp.halifax.rwth-aachen.de/fedora/linux/updates/40/Everything/x86_64
-    - https://mirror.23m.com/fedora/linux/updates/40/Everything/x86_64
-    - https://ftp.plusline.net/fedora/linux/updates/40/Everything/x86_64
-    - https://storage.googleapis.com/monogon-infra-public/mirror/fedora/linux/updates/40/Everything/x86_64
+    - https://dl.fedoraproject.org/pub/fedora/linux/updates/40/Everything/x86_64