[client] ci: run all build combinations to completion

This is desirable because certain uncommon configurations like libdecor or
clang may break, and this shouldn't stop us from seeing if unrelated changes
pass.

.github/workflows/build.yml

@@ -4,6 +4,7 @@ jobs:
   client:
     runs-on: ubuntu-20.04
     strategy:
+      fail-fast: false
       matrix:
         compiler:
           - {cc: gcc, cxx: g++}