Rollup merge of #52434 - Mark-Simulacrum:incremental-keep-stage, r=alexcrichton

Enable incremental independent of stage

Previously we'd only do so for stage 0 but with keep-stage
improvements it seems likely that we'll see more developers working in
the stage 1, so we should allow enabling incremental for them.

src/bootstrap/builder.rs

@@ -903,10 +903,7 @@ impl<'a> Builder<'a> {
                 .env("RUSTC_SNAPSHOT_LIBDIR", self.rustc_libdir(compiler));
         }
 
-        // Ignore incremental modes except for stage0, since we're
-        // not guaranteeing correctness across builds if the compiler
-        // is changing under your feet.`
-        if self.config.incremental && compiler.stage == 0 {
+        if self.config.incremental {
             cargo.env("CARGO_INCREMENTAL", "1");
         }