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

(cherry picked from commit 89684601)

.gitlab-ci.yml

@@ -22,6 +22,8 @@ variables:
   CARGO_HOME: /shared-storage/picodata/.cargo
   CACHE_PATHS: target .venv picodata-webui/node_modules
   CACHE_ARCHIVE: /shared-storage/picodata/cache.tar
+  # Helps to tolerate spurious network failures
+  GET_SOURCES_ATTEMPTS: 3
 
 # job:rules explained:
 #