ci: fix private roachtest nightly

[srosenberg]

It appears a change to TC wrapper scripts in [1] may have caused the root var. to become unbound. This PR adds the missing source statement.

[1] https://github.com/cockroachdb/cockroach/pull/124592

Epic: none

Release note: None

[cockroach-teamcity]

This change is 

[srosenberg]

Confirmed the fix by triggering a run.

[srosenberg]

TFTR!

bors r=darrylwong

[craig[bot]]

Build succeeded:

• unit_tests
• docker_image_amd64
• local_roachtest
• linux_amd64_fips_build
• local_roachtest_fips
• macos_amd64_build
• acceptance
• windows_build
• linux_arm64_build
• examples_orms
• lint
• macos_arm64_build
• check_generated_code
• linux_amd64_build

[blathers-crl[bot]]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from c8bc8076cee973d492d4a4d5c968d3f6d6a60e86 to blathers/backport-release-24.1-135913: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 24.1.x failed. See errors above.

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_{:owl: Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}