do not trust download-rustc=if-unchanged on CI for now

Author: RalfJung

src/ci/run.sh

@@ -179,7 +179,9 @@ else
   fi
 
   if [ "$NO_DOWNLOAD_CI_RUSTC" = "" ]; then
-    RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --set rust.download-rustc=if-unchanged"
+    # disabled for now, see https://github.com/rust-lang/rust/issues/131658
+    #RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --set rust.download-rustc=if-unchanged"
+    true
   fi
 fi