Fix CI: Use pre-installed Rust from rust:trixie container

- Remove unnecessary rustup installation since rust:trixie has Rust pre-installed
- Simplify to just verify the existing Rust toolchain
- Avoid conflicts with container's built-in Rust installation
- Should resolve the 'cannot install while Rust is installed' error

.forgejo/workflows/ci.yml

@@ -71,18 +71,10 @@ jobs:
           cp -r /tmp/apt-ostree/* .
           cp -r /tmp/apt-ostree/.* . 2>/dev/null || true
 
-      - name: Install Rust toolchain
+      - name: Verify Rust toolchain
         run: |
-          # Install rustup to get a stable Rust version
-          curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y
-          . ~/.cargo/env
-          
-          # Use a more stable Rust version (1.75.0 is known to be very stable)
-          rustup install 1.75.0
-          rustup default 1.75.0
-          
-          # Verify Rust version
-          echo "Using Rust version:"
+          # Rust is already installed in rust:trixie container
+          echo "Using pre-installed Rust version:"
           rustc --version
           cargo --version