Prepare for merging from rust-lang/rust

This updates the rust-version file to 4e391cf2425cf96521af17ff460e9f220e9bca00.

src/tools/miri/rust-version

diff --git a/src/tools/miri/rust-version b/src/tools/miri/rust-version
index 387bd8e..d113c79 100644
--- a/src/tools/miri/rust-version
+++ b/src/tools/miri/rust-version
@@ -1 +1 @@
-029c9e18dd1f4668e1d42bb187c1c263dfe20093
+4e391cf2425cf96521af17ff460e9f220e9bca00