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

This reverts commit b3c2121.

Author: onur-ozkan

src/ci/run.sh

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