fix: compilation failure without any async runtime

[snejugal]

cacache currently doesn't compile with both tokio and async-std disabled due to improper cfgs:

$ cargo c --no-default-features
    Checking cacache v13.1.0 (/home/snejugal/projects/cacache-rs)
error[E0412]: cannot find type `State` in this scope
   --> src/content/write.rs:430:37
    |
430 | fn update_state(current_state: &mut State, next_state: State) {
    |                                     ^^^^^ not found in this scope

error[E0412]: cannot find type `State` in this scope
   --> src/content/write.rs:430:56
    |
430 | fn update_state(current_state: &mut State, next_state: State) {
    |                                                        ^^^^^ not found in this scope

warning: unused import: `crate::Error`
  --> src/content/write.rs:22:5
   |
22 | use crate::Error;
   |     ^^^^^^^^^^^^
   |
   = note: `#[warn(unused_imports)]` on by default

For more information about this error, try `rustc --explain E0412`.
warning: `cacache` (lib) generated 1 warning
error: could not compile `cacache` (lib) due to 2 previous errors; 1 warning emitted

I guess it's worth testing this case in CI (since README shows how to use this crate sync-only), but I believe this belongs to a different PR.