Auto merge of #113298 - tgross35:update-bless-envs, r=oli-obk

Unite bless environment variables under `RUST_BLESS`

Currently, Clippy and Miri both 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:

- Rename the variable `MIRI_BLESS` (as used in the Miri subtree) to `RUST_BLESS`
- Rename the variable `BLESS` (as used in the Clippy subtree) to `RUST_BLESS`
- Move emitting `RUST_BLESS` into `prepare_cargo_test` so it is always available (I need this for a WIP PR)

---

I prefer something like `RUST_BLESS` to `BLESS` just for a lower chance of conflict (not super common but other tools [do use `BLESS`](https://grep.app/search?q=%22BLESS%22&case=true&words=true&filter[lang][0]=Text&filter[lang][1]=Rust&filter[lang][2]=Python&filter[lang][3]=C%2B%2B&filter[lang][4]=Markdown&filter[lang][5]=C&filter[lang][6]=JSON)), but I can change it to whatever is preferred.

Original discussion: https://rust-lang.zulipchat.com/#narrow/stream/257328-clippy/topic/BLESS.20env.20var.3A.20rename.20to.20CLIPPY_BLESS

r? `@oli-obk`
cc `@flip1995`

tests/compile-test.rs

@@ -115,7 +115,9 @@ fn base_config(test_dir: &str) -> compiletest::Config {
         mode: TestMode::Yolo,
         stderr_filters: vec![],
         stdout_filters: vec![],
-        output_conflict_handling: if var_os("BLESS").is_some() || env::args().any(|arg| arg == "--bless") {
+        // FIXME(tgross35): deduplicate bless env once clippy can update
+        output_conflict_handling: if var_os("RUSTC_BLESS").is_some_and(|v| v != "0")
+        || env::args().any(|arg| arg == "--bless") {
             compiletest::OutputConflictHandling::Bless
         } else {
             compiletest::OutputConflictHandling::Error("cargo test -- -- --bless".into())