do not build cargo-miri by default on stable channel

miri skipped for stable channel by default, do the same for `cargo-miri`.

Signed-off-by: onur-ozkan <work@onurozkan.dev>

Author: onur-ozkan

src/bootstrap/src/core/build_steps/tool.rs

@@ -1098,7 +1098,7 @@ tool_extended!((self, builder),
     CargoClippy, "src/tools/clippy", "cargo-clippy", stable=true;
     Clippy, "src/tools/clippy", "clippy-driver", stable=true, add_bins_to_sysroot = ["clippy-driver", "cargo-clippy"];
     Miri, "src/tools/miri", "miri", stable=false, add_bins_to_sysroot = ["miri"];
-    CargoMiri, "src/tools/miri/cargo-miri", "cargo-miri", stable=true, add_bins_to_sysroot = ["cargo-miri"];
+    CargoMiri, "src/tools/miri/cargo-miri", "cargo-miri", stable=false, add_bins_to_sysroot = ["cargo-miri"];
     // FIXME: tool_std is not quite right, we shouldn't allow nightly features.
     // But `builder.cargo` doesn't know how to handle ToolBootstrap in stages other than 0,
     // and this is close enough for now.