change (dockerfile): return to workaround sccache's bug

[TriplEight]

Still waiting until mozilla/sccache#663 will be merged, having

    Compiling futures-channel v0.3.5
  error[E0514]: found crate `hash_db` compiled by an incompatible version of rustc
    --> /usr/local/cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/trie-root-0.16.0/src/lib.rs:41:9
     |
  41 | pub use hash_db::Hasher;
     |         ^^^^^^^
     |
     = help: please recompile that crate using this compiler (rustc 1.49.0-nightly (b20253260 2020-11-01))
     = note: the following crate versions were found:
             crate `hash_db` compiled by rustc 1.49.0-nightly (ffe52882e 2020-10-30): /builds/parity/substrate/target/release/wbuild/target/wasm32-unknown-unknown/release/deps/libhash_db-7abf16653d25fc04.rmeta

randomly.