CI/build improvements

- use cache restore keys so that there's some caching even after deps
  change
- don't install ffmpeg stuff that hasn't been needed in a while

.github/workflows/ci.yml

@@ -28,9 +28,12 @@ jobs:
             ~/.cargo/registry
             ~/.cargo/git
             server/target
-          key: ${{ matrix.rust }}-cargo-${{ hashFiles('**/Cargo.lock') }}
+          key: cargo-${{ matrix.rust }}-${{ hashFiles('**/Cargo.lock') }}
+          restore-keys: |
+            cargo-${{ matrix.rust }}-
+            cargo-
       - name: Install dependencies
-        run: sudo apt-get update && sudo apt-get install libavcodec-dev libavformat-dev libavutil-dev libncurses-dev libsqlite3-dev pkgconf
+        run: sudo apt-get update && sudo apt-get install libncurses-dev libsqlite3-dev pkgconf
       - name: Install Rust
         uses: actions-rs/toolchain@v1
         with:

guide/build.md

@@ -184,9 +184,6 @@ most non-Rust dependencies:
 ```console
 $ sudo apt-get install \
                build-essential \
-               libavcodec-dev \
-               libavformat-dev \
-               libavutil-dev \
                libncurses-dev \
                libsqlite3-dev \
                pkgconf \