Revert "Enable incremental independent of stage"

This reverts commit 827f656.

Incremental is not sound to use across stages. Arbitrary changes to the compiler can invalidate the
incremental cache - even changes to normal queries, not incremental itself! - and we do not
currently enable `incremental-verify-ich` in bootstrap. Since 2018, we highly recommend and nudge
users towards stage 1 builds instead of stage 2, and using `keep-stage` for anything other than
libstd is very rare.

I don't think the risk of unsoundness is worth the very minor speedup when building libstd. Disable
incremental to avoid spurious panics and miscompilations when building with the stage 1 and 2
sysroot.

Author: jyn514

src/bootstrap/builder.rs

@@ -1783,7 +1783,10 @@ impl<'a> Builder<'a> {
             cargo.env("RUSTC_TLS_MODEL_INITIAL_EXEC", "1");
         }
 
-        if self.config.incremental {
+        // 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 {
             cargo.env("CARGO_INCREMENTAL", "1");
         } else {
             // Don't rely on any default setting for incr. comp. in Cargo