CI: rfl: support Rust >= 1.91.0 target spec

To unblock the Rust CI in PR [1], use a temporary commit from Rust for
Linux that supports the future target spec format.

Link: rust-lang#144443 [1]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

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

@@ -2,7 +2,8 @@
 
 set -euo pipefail
 
-LINUX_VERSION=v6.16-rc1
+# https://github.com/rust-lang/rust/pull/144443
+LINUX_VERSION=7770d51bce622b13195b2d3c85407282fc9c27e5
 
 # Build rustc, rustdoc, cargo, clippy-driver and rustfmt
 ../x.py build --stage 2 library rustdoc clippy rustfmt