Auto merge of #2283 - RalfJung:color, r=RalfJung

revert --color=always changes

They [cause problems](https://github.com/rust-lang/miri/issues/2277) and they completely break rendering on the playground:
```
   Compiling playground v0.0.1 (/playground)
    Finished dev [unoptimized + debuginfo] target(s) in 0.47s
     Running `/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/cargo-miri target/miri/x86_64-unknown-linux-gnu/debug/playground`
\e[0m\e[1m\e[38;5;9merror[E0277]\e[0m\e[0m\e[1m: `*mut std::sync::atomic::AtomicUsize` cannot be sent between threads safely\e[0m
\e[0m   \e[0m\e[0m\e[1m\e[38;5;12m--> \e[0m\e[0msrc/main.rs:17:18\e[0m
\e[0m    \e[0m\e[0m\e[1m\e[38;5;12m|\e[0m
\e[0m\e[1m\e[38;5;12m17\e[0m\e[0m  \e[0m\e[0m\e[1m\e[38;5;12m|\e[0m\e[0m   \e[0m\e[0m        let j1 = spawn(move || {\e[0m
\e[0m    \e[0m\e[0m\e[1m\e[38;5;12m| \e[0m\e[0m \e[0m\e[0m\e[1m\e[38;5;12m__________________\e[0m\e[0m\e[1m\e[38;5;9m^^^^^\e[0m\e[0m\e[1m\e[38;5;12m_-\e[0m
\e[0m    \e[0m\e[0m\e[1m\e[38;5;12m| \e[0m\e[0m\e[1m\e[38;5;12m|\e[0m\e[0m                  \e[0m\e[0m\e[1m\e[38;5;9m|\e[0m
\e[0m    \e[0m\e[0m\e[1m\e[38;5;12m| \e[0m\e[0m\e[1m\e[38;5;12m|\e[0m\e[0m                  \e[0m\e[0m\e[1m\e[38;5;9m`*mut std::sync::atomic::AtomicUsize` cannot be sent between threads safely\e[0m
\e[0m\e[1m\e[38;5;12m18\e[0m\e[0m  \e[0m\e[0m\e[1m\e[38;5;12m|\e[0m\e[0m \e[0m\e[0m\e[1m\e[38;5;12m|\e[0m\e[0m \e[0m\e[0m            *(c.0 as *mut usize) = 32;\e[0m
\e[0m\e[1m\e[38;5;12m19\e[0m\e[0m  \e[0m\e[0m\e[1m\e[38;5;12m|\e[0m\e[0m \e[0m\e[0m\e[1m\e[38;5;12m|\e[0m\e[0m \e[0m\e[0m        });\e[0m
\e[0m    \e[0m\e[0m\e[1m\e[38;5;12m| \e[0m\e[0m\e[1m\e[38;5;12m|_________-\e[0m\e[0m \e[0m\e[0m\e[1m\e[38;5;12mwithin this `[closure@src/main.rs:17:24: 19:10]`\e[0m
\e[0m    \e[0m\e[0m\e[1m\e[38;5;12m|\e[0m
```

Sorry `@saethlin,` I think we need to go back to start here and consider another solution.

Fixes https://github.com/rust-lang/miri/issues/2277