Small fix to the CI documentation that was missed in #35441
REPO_USE_WARP_RUNNERS is no longer there so can't be used.
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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35548.
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| ACK | willcl-ark |
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cc @willcl-ark
ACK 744d49501905144b4cef2c14efce37648130d078
lgtm