ci: switch mirrors to use our CDN

We recently added a CDN in front of our CI mirrors as it's faster and
cheaper for us. This switches libc's CI to use it instead of accessing
the underlying bucket directly.

Author: pietroalbini

ci/docker/mipsel-unknown-linux-musl/Dockerfile

@@ -16,10 +16,10 @@ RUN mkdir /toolchain
 
 # Note that this originally came from:
 # https://downloads.openwrt.org/snapshots/trunk/malta/generic/OpenWrt-Toolchain-malta-le_gcc-5.3.0_musl-1.1.15.Linux-x86_64.tar.bz2
-RUN curl -L https://s3-us-west-1.amazonaws.com/rust-lang-ci2/libc/OpenWrt-Toolchain-malta-le_gcc-5.3.0_musl-1.1.15.Linux-x86_64.tar.bz2 | \
+RUN curl -L https://ci-mirrors.rust-lang.org/libc/OpenWrt-Toolchain-malta-le_gcc-5.3.0_musl-1.1.15.Linux-x86_64.tar.bz2 | \
       tar xjf - -C /toolchain --strip-components=2
 
 ENV PATH=$PATH:/rust/bin:/toolchain/bin \
     CC_mipsel_unknown_linux_musl=mipsel-openwrt-linux-gcc \
     CARGO_TARGET_MIPSEL_UNKNOWN_LINUX_MUSL_LINKER=mipsel-openwrt-linux-gcc \
-    CARGO_TARGET_MIPSEL_UNKNOWN_LINUX_MUSL_RUNNER="qemu-mipsel -L /toolchain"
+    CARGO_TARGET_MIPSEL_UNKNOWN_LINUX_MUSL_RUNNER="qemu-mipsel -L /toolchain"