Remove Nix from CI, use rustup with caching

Install required system packages and set RUST_VERSION=1.99.1. Cache the
Rust
toolchain and Cargo registry, and run Cargo commands directly instead of
via
`nix develop`. Documentation files are updated to reflect the new
workflow.

.gitea/workflows/rust.yml

@@ -8,24 +8,51 @@ jobs:
   check:
     runs-on: ubuntu-latest
     container: node:22-bookworm
+    env:
+      RUST_VERSION: 1.99.1
+      CARGO_HOME: /root/.cargo
+      RUSTUP_HOME: /root/.rustup
     steps:
       - name: Checkout
         uses: https://github.com/actions/checkout@v4
-      - name: Install runner prerequisites
+      - name: Install build dependencies
         run: |
           apt-get update
-          apt-get install -y --no-install-recommends sudo
-      - name: Install Nix
-        uses: https://github.com/cachix/install-nix-action@v31
+          apt-get install -y --no-install-recommends ca-certificates curl build-essential pkg-config libasound2-dev zstd
+      - name: Rust toolchain cache
+        uses: https://github.com/actions/cache@v4
         with:
-          enable_kvm: false
+          path: |
+            /root/.rustup
+            /root/.cargo/bin
+          key: rust-toolchain-v1-linux-x64-${{ env.RUST_VERSION }}
+      - name: Install Rust
+        run: |
+          if [ ! -x "$CARGO_HOME/bin/rustup" ]; then
+            curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs -o /tmp/rustup-init.sh
+            sh /tmp/rustup-init.sh -y --profile minimal --default-toolchain none --no-modify-path
+          fi
+          "$CARGO_HOME/bin/rustup" toolchain install "$RUST_VERSION" --profile minimal --component rustfmt --component clippy
+          "$CARGO_HOME/bin/rustup" default "$RUST_VERSION"
+          echo "$CARGO_HOME/bin" >> "$GITHUB_PATH"
+      - name: Cargo cache
+        uses: https://github.com/actions/cache@v4
+        with:
+          path: |
+            /root/.cargo/registry
+            /root/.cargo/git
+            target
+          key: cargo-v1-linux-x64-${{ env.RUST_VERSION }}-${{ hashFiles('Cargo.lock') }}-${{ hashFiles('Cargo.toml', 'src/**', 'tests/**') }}
+          restore-keys: |
+            cargo-v1-linux-x64-${{ env.RUST_VERSION }}-${{ hashFiles('Cargo.lock') }}-
+            cargo-v1-linux-x64-${{ env.RUST_VERSION }}-
       - name: Formatting
-        run: nix develop --no-update-lock-file -c cargo fmt --all -- --check
+        run: cargo fmt --all -- --check
       - name: Compilation
-        run: nix develop --no-update-lock-file -c cargo check --all-targets --locked
+        run: cargo check --all-targets --locked
       - name: Tests
-        run: nix develop --no-update-lock-file -c cargo test --all-targets --locked
+        run: cargo test --all-targets --locked
       - name: Release tests
-        run: nix develop --no-update-lock-file -c cargo test --all-targets --release --locked
+        run: cargo test --all-targets --release --locked
       - name: Clippy
-        run: nix develop --no-update-lock-file -c cargo clippy --all-targets --locked
+        run: cargo clippy --all-targets --locked