ci: make ubuntu:devel and fedora:rawhide not to fail the pipeline

We will still attempt to build and run these two, with both gcc and
clang, but in case they fail, the CI pipeline will not fail and cancel
the other jobs.

It seems it will still mark the build with a red flag, though:
https://github.com/actions/toolkit/issues/399

.github/workflows/build.yml

@@ -6,19 +6,32 @@ on: [push, pull_request]
 jobs:
   linux:
     runs-on: ubuntu-18.04
+    continue-on-error: ${{ ! matrix.stable }}
     strategy:
       matrix:
         compiler:
           - gcc
           - clang
         os:
-          - fedora:rawhide
           - fedora:latest
           - centos:8
           - debian:testing
           - debian:latest
-          - ubuntu:devel
           - ubuntu:rolling
+        stable:: [true]
+        include:
+          - compiler: gcc
+            os: fedora:rawhide
+            stable: false
+          - compiler: clang
+            os: fedora:rawhide
+            stable: false
+          - compiler: gcc
+            os: ubuntu:devel
+            stable: false
+          - compiler: clang
+            os: ubuntu:devel
+            stable: false
     steps:
       - uses: actions/checkout@v2