ci: guard sources.list sed against missing file

Newer Ubuntu/Debian runners use /etc/apt/sources.list.d/ and don't have
/etc/apt/sources.list, causing the sed to fail with exit code 2.

.forgejo/workflows/e2e-tests.yaml

@@ -90,7 +90,7 @@ jobs:
       - name: Install system dependencies
         run: |
           . /etc/os-release
-          if [ "$ID" = "debian" ]; then
+          if [ "$ID" = "debian" ] && [ -f /etc/apt/sources.list ]; then
             sed -i 's/ main$/ main contrib non-free non-free-firmware/' /etc/apt/sources.list
           fi
           apt-get update

.forgejo/workflows/pr-tests.yaml

@@ -71,7 +71,7 @@ jobs:
       - name: Install system dependencies
         run: |
           . /etc/os-release
-          if [ "$ID" = "debian" ]; then
+          if [ "$ID" = "debian" ] && [ -f /etc/apt/sources.list ]; then
             sed -i 's/ main$/ main contrib non-free non-free-firmware/' /etc/apt/sources.list
           fi
           apt-get update

.forgejo/workflows/production.yaml

@@ -65,7 +65,7 @@ jobs:
       - name: Install system dependencies
         run: |
           . /etc/os-release
-          if [ "$ID" = "debian" ]; then
+          if [ "$ID" = "debian" ] && [ -f /etc/apt/sources.list ]; then
             sed -i 's/ main$/ main contrib non-free non-free-firmware/' /etc/apt/sources.list
           fi
           apt-get update