fix(ci): set DEBIAN_FRONTEND=noninteractive for apt-get installs

apt-get install was probing for a Dialog then Readline debconf frontend
before falling back to Teletype on the non-interactive CI runner,
adding noise and failed-negotiation log lines to every run. Passing
DEBIAN_FRONTEND=noninteractive directly on the sudo command line (env
vars set via step-level `env:` don't survive sudo's env_reset) skips
the negotiation and goes straight to the frontend that actually works
here.

.github/workflows/ci.yml

@@ -30,10 +30,10 @@ jobs:
       - name: Install fish
         run: |
           sudo apt-get update -qq
-          sudo apt-get install -y software-properties-common
+          sudo DEBIAN_FRONTEND=noninteractive apt-get install -y software-properties-common
           sudo add-apt-repository -y ppa:fish-shell/release-4
           sudo apt-get update -qq
-          sudo apt-get install -y fish
+          sudo DEBIAN_FRONTEND=noninteractive apt-get install -y fish
 
       - name: Run fish config tests
         run: fish tests/run-tests.fish
@@ -50,10 +50,10 @@ jobs:
       - name: Install dependencies
         run: |
           sudo apt-get update -qq
-          sudo apt-get install -y software-properties-common
+          sudo DEBIAN_FRONTEND=noninteractive apt-get install -y software-properties-common
           sudo add-apt-repository -y ppa:fish-shell/release-4
           sudo apt-get update -qq
-          sudo apt-get install -y pandoc python3-yaml fish
+          sudo DEBIAN_FRONTEND=noninteractive apt-get install -y pandoc python3-yaml fish
 
       - name: Generate concatenated markdown
         run: python3 docs/build-manual.py --concat -o docs/fish-config.md