ci: do not cancel all jobs of a matrix if one fails

ci: do not cancel all jobs of a matrix if one fails

The CI/PR GitHub Actions workflow uses the 'matrix' strategy for the
"windows-test", "vs-test", "regular" and "dockerized" jobs. The default
behaviour of GitHub Actions is to cancel all in-progress jobs in a
matrix if one of the job of the matrix fails [1].

This is not ideal as a failure early in a job, like during installation of
the build/test dependencies on a specific platform, leads to the
cancellation of all other jobs in the matrix.

Set the 'fail-fast' variable to 'false' for all four matrix jobs in the
workflow.

[1] https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#jobsjob_idstrategyfail-fast

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>