This fixes the reproducibility of //rs/ic_os/config:config in #5264 by using the same trick as used in rs/http_endpoints/public/build.rs.

The reason it was non-reproducible before:

1. The askama macro calls the include_bytes!(#path) macro where path will be an absolute path to the template which will be unique for every Bazel invocation
2. The output of proc_macros will be hashed and end up in the crate's metadata hash.
3. Since the output of the askama macro is different for every Bazel invocation the crate's metadata hash will be different every time.
4. The crate metadata hash is an input to generated symbol names during code generation.
5. This means that we get different symbol names for every Bazel invocation. This can be observed when running diffoscope on the binaries.