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This PR bumps the rust toolchain in CI to resolve the CI build issue

```bash
  Downloaded crossbeam-utils v0.8.19
  Downloaded crc32fast v1.3.2
error: failed to compile `text-generation-router v1.4.0 (/home/runner/work/text-generation-inference/text-generation-inference/router)`, intermediate artifacts can be found at `/home/runner/work/text-generation-inference/text-generation-inference/target`

Caused by:
  package `clap_lex v0.7.0` cannot be built because it requires rustc 1.74 or newer, while the currently active rustc version is 1.71.0
  Either upgrade to rustc 1.74 or newer, or use
  cargo update -p clap_lex@0.7.0 --precise ver
  where `ver` is the latest version of `clap_lex` supporting rustc 1.71.0
make: *** [Makefile:12: install-router] Error 101
```