ci: use our own mirror for fuchsia's zircon repository

The canonical repository on fuchsia.googlesource.com is not accessible
anymore, neither for anonymous access nor logged in access. This commit
switches our CI to fetch the repository from our own mirror.

Author: pietroalbini

src/ci/docker/dist-various-2/build-fuchsia-toolchain.sh

@@ -10,7 +10,7 @@ pushd zircon > /dev/null
 
 # Download sources
 git init
-git remote add origin https://fuchsia.googlesource.com/zircon
+git remote add origin https://github.com/rust-lang-nursery/mirror-google-fuchsia-zircon
 git fetch --depth=1 origin $ZIRCON
 git reset --hard FETCH_HEAD