Update bazel toolchain mirror (#29980)
Update bazel toolchain mirror (#29980)

See https://github.com/bazelbuild/bazel-toolchains/issues/891#issuecomment-654877498
and https://github.com/grpc/grpc/pull/27410#discussion_r715420745

Seems like the mirror for bazel toolchains changed the URL and they
mirror downloads but not archives. This should fix any issues with
missing mirror and workaround https://github.com/bazelbuild/bazel-toolchains/issues/972

Signed-off-by: Vihang Mehta <a47ecd7a5be7797dc12a126288bdefa499ee9afa@pixielabs.ai>
stanley-cheung/grpc,jtattermusch/grpc,jtattermusch/grpc,grpc/grpc,stanley-cheung/grpc,ctiller/grpc,jtattermusch/grpc,grpc/grpc,ejona86/grpc,ejona86/grpc,jtattermusch/grpc,stanley-cheung/grpc,jtattermusch/grpc,ejona86/grpc,stanley-cheung/grpc,grpc/grpc,stanley-cheung/grpc,ejona86/grpc,ctiller/grpc,ejona86/grpc,ejona86/grpc,grpc/grpc,ejona86/grpc,grpc/grpc,ctiller/grpc,jtattermusch/grpc,grpc/grpc,stanley-cheung/grpc,jtattermusch/grpc,jtattermusch/grpc,grpc/grpc,ctiller/grpc,ctiller/grpc,ctiller/grpc,ejona86/grpc,jtattermusch/grpc,grpc/grpc,ejona86/grpc,ctiller/grpc,ctiller/grpc,jtattermusch/grpc,stanley-cheung/grpc,grpc/grpc,grpc/grpc,ctiller/grpc,stanley-cheung/grpc,grpc/grpc,ejona86/grpc,ctiller/grpc,ctiller/grpc,stanley-cheung/grpc,stanley-cheung/grpc,grpc/grpc,ctiller/grpc,stanley-cheung/grpc,jtattermusch/grpc,ejona86/grpc,stanley-cheung/grpc,ejona86/grpc,jtattermusch/grpc