feat: Force use of pip3 to install python packages.

[dianakhuang]

This should be a no-op in most cases, but for older installations where it isn't the default, forces the use.

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Make sure that the following steps are done before merging:

• Have a Site Reliability Engineer review the PR if you don't own all of the services impacted.
• If you are adding any new default values that need to be overridden when this change goes live, update internal repos and add an entry to the top of the CHANGELOG.
• Performed the appropriate testing.