Move GIT_CREDENTIAL_STORE_SCRIPT from variables volume to scripts configmap

MR: Pending

Description

Followup from !182354 (comment 2481242318) on Convert existing Workspaces internal script log... (!182354 - merged) • Chad Woolley • 18.0

See TODO in ee/lib/remote_development/workspace_operations/create/workspace_variables_builder.rb:

            {
              # TODO: We should be able to move this entry to the scripts volume.
              # This script is set as the value of `credential.helper` below in `GIT_CONFIG_VALUE_0`
              key: GIT_CREDENTIAL_STORE_SCRIPT_FILE_NAME,
              value: GIT_CREDENTIAL_STORE_SCRIPT,
              variable_type: FILE_TYPE,
              workspace_id: workspace_id
            },

Acceptance criteria

• Script is moved from variables volume to scripts configmap volume

Implementation plan

• Move it, make sure tests pass, and workspaces still function.