chore: retry checkouts on CI

After seeing failed checkout here
https://git.picodata.io/picodata/picodata/picodata/-/jobs/106805
I've found that gitlab supports specifying number of retries per job
stage. Use 3 attempts before giving up on fetching sources. See
https://docs.gitlab.com/ee/ci/runners/configure_runners.html#job-stages-attempts
for details