chore: Use rust:bullseye as Docker builder

Use `bullseye` explicitly to make the correspondence with the final
image clear, and remove explicit Rust version as it does not seem that
important, and the final runtime image version is what it is at the time
too.

Dockerfile

@@ -1,4 +1,4 @@
-FROM rust:1.65.0 as builder
+FROM rust:bullseye as builder
 WORKDIR /usr/src/typos
 COPY . .
 RUN cargo install --path .