Currently, Clippy, Miri, Rustfmt, and rustc all use an environment variable to
indicate that output should be blessed, but they use different variable names.
In order to improve consistency, this patch applies the following changes:
- Emit `RUSTC_BLESS` within `prepare_cargo_test` so it is always
available
- Change usage of `MIRI_BLESS` in the Miri subtree to use `RUSTC_BLESS`
- Change usage of `BLESS` in the Clippy subtree to `RUSTC_BLESS`
- Change usage of `BLESS` in the Rustfmt subtree to `RUSTC_BLESS`
- Adjust the blessable test in `rustc_errors` to use this same
convention
- Update documentation where applicable
Any tools that uses `RUSTC_BLESS` should check that it is set to any value
other than `"0"`.
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