CI: Use fail-fast: false for coverage jobs.

Frequently lately one coverage job has been failing per job. Instead of
re-running the whole matrix again, make it so we just need to run the
(usually) one job that failed.

.github/workflows/ci.yml

@@ -478,6 +478,7 @@ jobs:
     runs-on: ${{ matrix.host_os }}
 
     strategy:
+      fail-fast: false
       matrix:
         features:
           # Enable all the library features so we can measure all the coverage.