Previously, clean only supported `--stage 0` for specific crates.

The new `crate_description` function generates a string that looks
like
```
: {rustc_query_impl}
```

Before:
```
Testing ["rustc_interface"] stage0 (aarch64-unknown-linux-gnu -> aarch64-unknown-linux-gnu)
```

After:
```
Testing {rustc_interface} stage0 (aarch64-unknown-linux-gnu -> aarch64-unknown-linux-gnu)
```

Note there is a slight consistency between `build` and `test`: The
former doesn't print "compiler artifacts". It would be annoying to fix
and doesn't hurt anything, so I left it be.

```
; x t rustc_interface --stage 0 --dry-run
Testing {rustc_interface} stage0 (aarch64-unknown-linux-gnu -> aarch64-unknown-linux-gnu)
; x b rustc_interface --stage 0 --dry-run
Building {rustc_interface} stage0 compiler artifacts (aarch64-unknown-linux-gnu -> aarch64-unknown-linux-gnu)
```