Fix Pipenv matrix flake on 503/DNS transport blips

[Mikola Lysenko (mikolalysenko)]

Problem

Pipenv compatibility is the flakiest PR workflow by re-run count. Between 2026-09-18 and 2026-10-01, 10 runs went red on attempt 1 and green on re-run of the same SHA, covering 15 failed matrix legs (9 on the 8-release ubuntu direct leg, 3 on the macOS leg, 3 on the ubuntu crlf/marker-excluded/extras/category leg). Each re-run costs a full matrix leg (5–7 min each, on top of the cargo build) plus a human noticing and re-running it.

Examples of red-then-green runs:

• https://github.com/SocketDev/socket-patch/actions/runs/36699362953 (3 legs red on attempt 1)