Current pinned version of `sel4-sys` fails to build with MacOS-supplied clang

[Willmish]

https://github.com/seL4/rust-root-task-demo and https://github.com/seL4/rust-microkit-http-server-demo would probably also benefit from an update.

In short, the examples (and some other project I was compiling) were pinned to the commit just after v1.0, so their builds do not include seL4/rust-sel4@88b5fe7 , so the issue with clang can fail in parsing the custom target name when building sel4-sys crate (and that happened to me on Mac)

More specifically, you get a panic:

.cargo/git/checkouts/rust-sel4-a4edf3b9624837c3/4f13dd2/crates/sel4/sys/build/c.rs:69:10:
  called `Result::unwrap()` on an `Err` value: ClangDiagnostic("error: version 'minimal' in target triple 'aarch64-sel4-microkit-minimal' is invalid\n")

suggested fix:

Bump Cargo.lock to most recent versions of rust-sel4. When building on MacOS natively (without docker) this example compiled successfully on the most recent commit: seL4/rust-sel4@5b9ebfd