fix the Rust toolchain in the CI (!195)

this is to avoid having to change the toolchain in the CI everytime.

`rust-toolchain.toml` and `.gitlab-ci.yml` will have to be updated in pairs when bumping / changing the Rust toolchain.

.gitlab-ci.yml

-image: "rust:latest"
+# WARNING: update `rust-toolchain.toml` as well
+image: "rust:1.78"
 
 stages:
   - fmt

rust-toolchain.toml

 [toolchain]
 profile = "minimal"
+# WARNING: update `.gitlab-ci.yml` as well
 channel = "1.78"
 components = ["rustfmt", "clippy", "rust-analyzer"]