Run rustup toolchain install explicitly (#6928)

rustup will no longer automatically install the active toolchain if it
is not installed. To ensure its installation, run `rustup toolchain
install` with no arguments.

https://blog.rust-lang.org/2025/03/02/Rustup-1.28.0.html



In the cloudbuild logs we see this happening which is breaking our
builds

```
info: default host triple is x86_64-unknown-linux-gnu
info: skipping toolchain installation
```

---------

Co-authored-by: getsantry[bot] <66042841+getsantry[bot]@users.noreply.github.com>

Dockerfile

@@ -91,6 +91,7 @@ COPY ./scripts/rust-dummy-build.sh ./scripts/rust-dummy-build.sh
 RUN set -ex; \
     sh scripts/rust-dummy-build.sh; \
     cd ./rust_snuba/; \
+    rustup show active-toolchain || rustup toolchain install; \
     maturin build --release --compatibility linux --locked
 
 FROM build_rust_snuba_base AS build_rust_snuba
@@ -99,6 +100,7 @@ COPY --from=build_rust_snuba_deps /usr/src/snuba/rust_snuba/target/ ./rust_snuba
 COPY --from=build_rust_snuba_deps /root/.cargo/ /root/.cargo/
 RUN set -ex; \
     cd ./rust_snuba/; \
+    rustup show active-toolchain || rustup toolchain install; \
     maturin build --release --compatibility linux --locked
 
 # Install nodejs and yarn and build the admin UI