Use default image for test-on-omnibus-env

test-on-omnibus-env pulls alpine through the group dependency proxy, which returns 404 when the pipeline runs as the project service account that opens dependency MRs. Every bot pipeline fails here until a person re-runs it.

ERROR: Job failed: failed to pull image "gitlab.com:443/gitlab-org/dependency_proxy/containers/alpine:latest" with specified policies [always]: Error response from daemon: error parsing HTTP 404 response body: unexpected end of JSON input: ""

Failing, as the service account:

• https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/17158143779 (!1466 (closed))
• https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/17184168177 (!1467 (merged))

Same MRs passing when a person runs the pipeline:

• https://gitlab.com/gitlab-org/gitlab-qa/-/pipelines/2940350280 (!1467 (merged))
• https://gitlab.com/gitlab-org/gitlab-qa/-/pipelines/2946024909 (!1472 (merged), after four failed bot pipelines)

The script needs only sh, md5sum and awk, so this drops the override and uses the default image.