Always permit miri to fail to build

This presumably "accidentally" never actually got triggered in the past, but
since we don't actually require shipping miri on beta or stable, this should be
fine.

Author: Mark-Simulacrum

src/bootstrap/dist.rs

@@ -1461,18 +1461,18 @@ impl Step for Miri {
         builder.create_dir(&image);
 
         // Prepare the image directory
-        // We expect miri to build, because we've exited this step above if tool
-        // state for miri isn't testing.
+        // We're always fine with miri not building, we don't ship it on
+        // beta/stable anyway.
         let miri = builder
             .ensure(tool::Miri { compiler, target, extra_features: Vec::new() })
             .or_else(|| {
-                missing_tool("miri", builder.build.config.missing_tools);
+                missing_tool("miri", true);
                 None
             })?;
         let cargomiri = builder
             .ensure(tool::CargoMiri { compiler, target, extra_features: Vec::new() })
             .or_else(|| {
-                missing_tool("cargo miri", builder.build.config.missing_tools);
+                missing_tool("cargo miri", true);
                 None
             })?;