CI(benchmarking): fix setting LD_LIBRARY_PATH

[bayandin]

Problem

pgbench-pgvector job fails with the error:

/__w/_temp/f45bc2eb-4c4c-4f0a-8030-99079303fa65.sh: line 17: LD_LIBRARY_PATH: unbound variable

Missed in #9178

Summary of changes

• Fix LD_LIBRARY_PATH: unbound variable error in benchmarks

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

5091 tests run: 4933 passed, 0 failed, 158 skipped (full report)

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Flaky tests (11)

Postgres 17

• test_pageserver_compaction_smoke: release-x86-64
• test_ondemand_wal_download_in_replication_slot_funcs: release-x86-64

Postgres 16

• test_ondemand_wal_download_in_replication_slot_funcs: release-x86-64
• test_slots_and_branching: release-x86-64

Postgres 15

• test_ondemand_wal_download_in_replication_slot_funcs: release-x86-64
• test_obsolete_slot_drop[cross-validation]: release-x86-64
• test_large_records[cross-validation]: release-x86-64
• test_subscriber_restart: release-x86-64

Postgres 14

• test_replica_start_scan_clog_crashed_xids: release-arm64
• test_subscriber_restart: release-x86-64
• test_get_tenant_size_with_multiple_branches: release-x86-64

Code coverage* (full report)

• functions: 31.5% (7486 of 23798 functions)
• lines: 49.8% (60108 of 120691 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
80211c2 at 2024-09-29T22:37:12.769Z :recycle:}