Stop cancelling build matrix when one build fails (#868)

Motivation:

Motivation:
We unfortunately have a number of flaky tests haunting our builds. The
GHA build matrix will by default cancel all matrix jobs when one job
experiences a failure. This is unproductive with flaky tests, because
the more failed builds that need to rerun, the higher the chances of
more failures. Also, seeing builds pass on other JVM versions helps
build confidence, even if other versions failed on a flaky test.

Modification:
Disable fail-fast on matrix-strategy builds.

Result:
Jobs in a matrix build no longer get cancelled if a sibling-job fails.

.github/workflows/ci-pr.yml

@@ -17,6 +17,7 @@ jobs:
   build-pr:
     runs-on: ubuntu-latest
     strategy:
+      fail-fast: false
       matrix:
         include:
           - setup: centos6-x86_64