ci: pin to ubuntu-20.04 for Gio's benefit

With the latest Gio updates, the headless API doesn't seem to render
stable images across machines and OS-stacks.
Pin to Ubuntu-20.04 for the time being.

Signed-off-by: Sebastien Binet <binet@cern.ch>

.github/workflows/ci.yml

@@ -22,7 +22,7 @@ jobs:
     strategy:
       matrix:
         go-version: [1.20.x, 1.19.x]
-        platform: [ubuntu-latest, macos-latest, windows-latest]
+        platform: [ubuntu-20.04, macos-latest, windows-latest]
     runs-on: ${{ matrix.platform }}
     steps:
     - name: Install Go
@@ -50,7 +50,7 @@ jobs:
         fetch-depth: 1
 
     - name: Install Linux packages
-      if: matrix.platform == 'ubuntu-latest'
+      if: matrix.platform == 'ubuntu-20.04'
       run: |
         sudo apt-get update
         sudo apt-get install -qq gcc pkg-config libwayland-dev libx11-dev libx11-xcb-dev libxkbcommon-x11-dev libgles2-mesa-dev libegl1-mesa-dev libffi-dev libxcursor-dev libvulkan-dev xvfb xdotool
@@ -59,11 +59,11 @@ jobs:
         root.exe --version
     
     - name: Build-Linux-32b
-      if: matrix.platform == 'ubuntu-latest'
+      if: matrix.platform == 'ubuntu-20.04'
       run: |
         GOARCH=386   go install -v $TAGS,cross_compile ./...
     - name: Build-Linux-64b
-      if: matrix.platform == 'ubuntu-latest'
+      if: matrix.platform == 'ubuntu-20.04'
       run: |
         GOARCH=amd64 go install -v $TAGS ./...       
     - name: Build-Windows
@@ -75,7 +75,7 @@ jobs:
       run: |
         go install -v $TAGS ./...
     - name: Test Linux
-      if: matrix.platform == 'ubuntu-latest'
+      if: matrix.platform == 'ubuntu-20.04'
       run: |
         go run ./ci/run-tests.go $TAGS -race $COVERAGE
     - name: Test Windows
@@ -93,5 +93,5 @@ jobs:
         cache-key: ${{ matrix.platform }}
         version: "2023.1"
     - name: Upload-Coverage
-      if: matrix.platform == 'ubuntu-latest'
+      if: matrix.platform == 'ubuntu-20.04'
       uses: codecov/codecov-action@v3