GitHub-CI/build_debs_Debian-unstable: make job outcome optional

Temporary ignore the outcome of building Smuxi on Debian/unstable as it
will always because unstable no longer contains the required
build-dependency GTK#2. This issue is tracked in:
#297

Hopefully GTK# and Smuxi can be brought back soon.

.github/workflows/auto-ci-builds.yml

@@ -300,6 +300,7 @@ jobs:
   build_debs_Debian-unstable:
     runs-on: ubuntu-latest
     container: debian:unstable
+    continue-on-error: true
     steps:
     - uses: actions/checkout@v1
     - name: check Debian version