fix(ci): force python3 symlink in Velox deps image

[Avinash-Raj]

Summary

• Patch upstream centos-multi.dockerfile after checkout so ln -s $(which python3) /usr/bin/python becomes ln -sfn $(which python3) /usr/bin/python.
• Unblocks every velox-deps matrix cell; without a deps image, velox-build, tests, and benchmarks never start.

Motivation

Nightly velox-nightly failed on all four deps jobs (cuda12.9/13.1 x amd64/arm64):

https://github.com/rapidsai/velox-testing/actions/runs/34085672229

RUN ln -s $(which python3) /usr/bin/python
ln: failed to create symbolic link '/usr/bin/python': File exists

That line is in facebookincubator/velox scripts/docker/centos-multi.dockerfile after install_build_prerequisites && install_velox_deps_from_dnf. On current CentOS Stream 9, /usr/bin/python can already exist (the RPM that owns it is python-unversioned-command, not python3-pip / python3-devel). Plain ln -s then fails.

ln -sfn creates the python -> python3 link when missing and replaces it when present. Same pattern as the existing ARM_BUILD_TARGET checkout patch. Drop this step once upstream makes the symlink idempotent.

Test plan

• velox-deps succeeds for cuda12.9/13.1 on amd64 and arm64
• Confirm the build log shows ln -sfn $(which python3) /usr/bin/python rather than ln -s
• Downstream velox-build is no longer skipped

[bdice]

Compare this to prestodb/presto#28434, it seems simpler.

[Avinash-Raj]

Yes, but we make sure that the base image symlinks python3 to /usr/bin/python

[Avinash-Raj]

Verified that the base image contain the /usr/bin/python symlink by default and it points to the python3 -> python3.9

$ docker run --rm quay.io/centos/centos:stream9 ls -la /usr/bin/python
lrwxrwxrwx 1 root root 9 Aug 14 13:30 /usr/bin/python -> ./python3

[Avinash-Raj]

Closing this since the fix was already landed on upstream https://github.com/facebookincubator/velox/blob/main/scripts/docker/centos-multi.dockerfile#L110