Auto merge of #57891 - michaelwoerister:no-default-incr-bootstrap, r=pietroalbini

bootstrap: Don't rely on any default settings regarding incr. comp. in Cargo

https://github.com/rust-lang/cargo/pull/6564 (temporarily) makes incremental compilation the default for release builds. We don't want this default to apply to building the compiler itself, that is, `bootstrap`'s `incremental` flag should always be respected. Otherwise we'll get incrementally build releases, which we really don't want `:)`.

r? @Mark-Simulacrum

Anybody else from @rust-lang/release should feel free to r+ this too if they get around to it earlier.

src/bootstrap/builder.rs

@@ -991,6 +991,9 @@ impl<'a> Builder<'a> {
 
         if self.config.incremental {
             cargo.env("CARGO_INCREMENTAL", "1");
+        } else {
+            // Don't rely on any default setting for incr. comp. in Cargo
+            cargo.env("CARGO_INCREMENTAL", "0");
         }
 
         if let Some(ref on_fail) = self.config.on_fail {