workload: reduce import concurrency in large schema benchmark

[spilchen]

Reduce the import concurrency in the large schema benchmark to mitigate system overload issues observed recently, particularly in the multi-region variant. Given recent changes to how clusters are initialized to avoid deadlocks, we probably don't need such a high concurrency anyway.

Closes #158947

Release note: None Epic: None

[cockroach-teamcity]

This change is 

[spilchen]

TFTR

bors r+

[craig[bot]]

Build failed:

• docker_image_amd64

[spilchen]

bors retry

[craig[bot]]

Build failed:

• check_generated_code

[spilchen]

bors retry

[craig[bot]]

Build failed (retrying...):

• unit_tests

[craig[bot]]

Build succeeded:

• lint
• examples_orms
• unit_tests
• local_roachtest
• acceptance
• check_generated_code
• linux_amd64_fips_build
• linux_amd64_build
• local_roachtest_fips
• docker_image_amd64