[CI] Azure.sonic-swss (Test vstest) Install dependencies: pip3 not found

[peterbailey-arista]

Azure pipelines builds are failing on test vstest at the install dependencies step

+ sudo pip3 install pytest flaky exabgp docker lcov_cobertura
sudo: pip3: command not found

seen in many or all recent PRs:
#4762
#4760
#4749

[pinky-nexthop]

@sonic-net/sonic-swss-maintainers this is still ongoing and blocking vstest / TestAsan vstest across the repo — no PR has produced a green vstest since the pool broke.

Confirmed still failing on today's builds, e.g. build 1169537 (2026-07-20 03:46 UTC), identical error at the "Install dependencies" step:

+ sudo pip3 install pytest flaky exabgp docker lcov_cobertura
sudo: pip3: command not found

Every recent build (buildId ≳ 1167xxx) fails here; the only PRs showing a passing vstest are older runs from before the regression. Looks like the test-pool agent image is missing pip3 on PATH. Could someone from the infra side take a look at the sonictest pool image? Thanks.
cc: @prsunny @vmittal-msft