Fix Python installation in CI

[tenzen-y]

Signed-off-by: Yuki Iwai yuki.iwai.tz@gmail.com

What this PR does / why we need it:
This PR fixes the following error in CI:

[integration-test (v1.25.2, none, 3.8)](https://github.com/kubeflow/training-operator/actions/runs/4166979917/jobs/7211993949#step:3:9)
The `python-version` input is not set. The version of Python currently in `PATH` will be used.

https://github.com/kubeflow/training-operator/actions/runs/4166979917

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):
Fixes #

Checklist:

• Docs included if any changes are user facing

[coveralls]

Pull Request Test Coverage Report for Build 4170321509

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 39.356%

---

Files with Coverage Reduction	New Missed Lines	%
pkg/controller.v1/mpi/mpijob_controller.go	2	76.97%

Totals
Change from base Build 4147261027:	-0.2%
Covered Lines:	2725
Relevant Lines:	6924

---

💛 - Coveralls

[johnugeorge]

Thanks @tenzen-y

/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: johnugeorge, tenzen-y

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