CI: rfl: move job forward to Linux v6.13-rc1

Linux v6.13-rc1 contains commit 28e848386b92 ("rust: block: fix formatting
of `kernel::block::mq::request` module"), which in turn contains commit
c95bbb59a9b2 ("rust: enable arbitrary_self_types and remove `Receiver`"),
which is why we had a hash rather than a tag.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

src/ci/docker/scripts/rfl-build.sh

@@ -2,7 +2,7 @@
 
 set -euo pipefail
 
-LINUX_VERSION=28e848386b92645f93b9f2fdba5882c3ca7fb3e2
+LINUX_VERSION=v6.13-rc1
 
 # Build rustc, rustdoc, cargo, clippy-driver and rustfmt
 ../x.py build --stage 2 library rustdoc clippy rustfmt