rustbuild: only plan from build triple for dist

We only want to package each host/target once for `dist`. The obvious
solution takes the form of step dependency, which is implemented at
least for the `dist-rustc` step. Unfortunately since the steps are
created from `hosts x targets` during planning and *not* de-duplicated
afterwards, the problem still persists.

We therefore move the check inside `plan()` instead, to avoid creating
the duplicate steps in the first place.

src/bootstrap/step.rs

@@ -822,7 +822,16 @@ invalid rule dependency graph detected, was a rule added and maybe typo'd?
             let hosts = if self.build.flags.host.len() > 0 {
                 &self.build.flags.host
             } else {
-                &self.build.config.host
+                if kind == Kind::Dist {
+                    // For 'dist' steps we only distribute artifacts built from
+                    // the build platform, so only consider that in the hosts
+                    // array.
+                    // NOTE: This relies on the fact that the build triple is
+                    // always placed first, as done in `config.rs`.
+                    &self.build.config.host[..1]
+                } else {
+                    &self.build.config.host
+                }
             };
             let targets = if self.build.flags.target.len() > 0 {
                 &self.build.flags.target