CI: launch xvfb

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

.github/workflows/ci.yml

@@ -8,6 +8,8 @@ jobs:
     # We want to run on external PRs, but not on our own internal PRs as they'll be run by the push
     # to the branch.
     if: github.event_name == 'push' || github.event.pull_request.head.repo.full_name != github.repository
+    env:
+      DISPLAY: ':99.0'
     steps:
       - name: Checkout
         uses: actions/checkout@v2
@@ -25,6 +27,8 @@ jobs:
           AGREE: true
       - name: x86_64-linux -> x86_64-windows
         run: zig build test -Dtarget=x86_64-windows
+      - name: launch xvfb
+        run: Xvfb :99 -screen 0 1680x720x24 > /dev/null 2>&1 &
       - name: mach::test
         run: |
           sudo add-apt-repository -y ppa:kisak/kisak-mesa