Disable test matrix fail-fast

Jobs were getting canceled when one would fail. Because we are testing
different Ruby versions, it's quite possible there's failures limited to
specific versions, all of which we need to know about.

.github/workflows/ci.yml

@@ -7,6 +7,7 @@ jobs:
     runs-on: ubuntu-latest
 
     strategy:
+      fail-fast: false
       matrix:
         rustup-toolchain:
           - stable