C2Rust: Use fixes needed with Jammy's LLVM

This uses a branch of the LLVM PR <immunant/c2rust#693>
which fixes C2Rust's issue <immunant/c2rust#692>.
Both have been preexisting for a while, but became critical when
switching over to the current LLVM version.

riotbuild/Dockerfile

@@ -344,7 +344,7 @@ RUN \
 
 RUN \
     echo 'Installing C2Rust' >&2 && \
-    CARGO_HOME=/opt/rustup/.cargo cargo install --no-track --locked c2rust --git https://github.com/immunant/c2rust --rev 7d88216756204553a44e2b1a39f840922229d5d6 && \
+    CARGO_HOME=/opt/rustup/.cargo cargo install --no-track --locked c2rust --git https://github.com/chrysn-pull-requests/c2rust --branch riscv-vector-types && \
     echo 'Cleaning up root-owned crates.io cache' >&2 && \
     rm -rf /opt/rustup/.cargo/{git,registry,.package-cache}