Fix and unify artifact collection in CI script

[rzetelskik]

Description of your changes: #1754 broke e2e artifacts collection in our CI. This PR fixes it by waiting for the e2e container to exit and then collecting the artifacts from its neighbour. It also removes the superfluous concurrency and unifies the collection of must-gather artifacts with e2e artifacts.

Which issue is resolved by this Pull Request:
Resolves #1910

/kind bug
/priority critical-urgent

[tnozicka]

/approve
/lgtm
thanks for fixing this

/hold
to check artifacts manually when the jobs finish

[scylla-operator-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rzetelskik, tnozicka

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[tnozicka]

/hold cancel
artifacts are present https://gcsweb.scylla-operator.scylladb.com/gcs/scylla-operator-prow/pr-logs/pull/scylladb_scylla-operator/1911/pull-scylla-operator-master-e2e-gke-serial/1787821240048685056/artifacts/

[tnozicka]

/lgtm

[tnozicka]

/lgtm cancel
I've tried to pick this into my PR but the exit code is incorrectly set even here

 + exit_code='I0507 12:39:52.777049       1 featuregate/feature_gate.go:249] feature gates: &{map[AllAlpha:true AllBeta:true AutomaticTLSCertificates:true]}
I0507 12:39:52.777228       1 tests/tests.go:30] maxprocs: Leaving GOMAXPROCS=[8]: CPU quota undefined 

those logs should be on stderr :(
https://prow.scylla-operator.scylladb.com/view/gs/scylla-operator-prow/pr-logs/pull/scylladb_scylla-operator/1911/pull-scylla-operator-master-e2e-gke-serial/1787821240048685056#1:test-build-log.txt%3A524

[tnozicka]

/lgtm
thanks

[rzetelskik]

@rzetelskik: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name Commit Details Required Rerun command
ci/prow/e2e-gke-parallel fa4b0ba link true /test e2e-gke-parallel
Full PR test history. Your PR dashboard.

known manager flake
/retest