Update Wasmtime's Rust toolchain (google#12930)

The default Rust toolchain use to build Rust projects on OSS-Fuzz,
`nightly-2024-07-12`, is too old to build Wasmtime at this time. This
toolchain is held back to match the LLVM version that Clang is using
(18) and is one of the last Rust nightly builds that still uses LLVM 18
(`nightly-2024-07-30` is the last Rust nightly using 18,
`nightly-2024-08-01` uses LLVM 19).

In lieu of updating the toolchain for all projects I've opted to only
update Wasmtime at this time. I believe this means that Wasmtime will
lose coverage information for fuzzing because LLVM tooling is at version
18 whereas Wasmtime will be producing coverage files from LLVM 19. For
Wasmtime this is ok as we rely on the fuzzers themselves much more than
the retrospective coverage information.

Author: alexcrichton

Diff for: projects/wasmtime/Dockerfile

@@ -36,4 +36,8 @@ WORKDIR wasmtime
 
 #RUN git clone --depth 1 https://github.com/bytecodealliance/wasmtime-libfuzzer-corpus wasmtime-libfuzzer-corpus
 
+# The default toolchain used by OSS-Fuzz is too old to build Wasmtime at this
+# time.
+ENV RUSTUP_TOOLCHAIN nightly-2025-01-10
+
 COPY build.sh *.options $SRC/