[skip ci] Clarify instructions about the python version that is being used to create a new local environment

[srherbener]

Summary

This PR adds some clarification that was discovered while debugging spack-stack builds on S4. See #1384 for details.

Testing

Documentation only update. Relying on the github check that builds the documentation.

Applications affected

None - documentation update for building a new local environment

Systems affected

All - documentation update for building a new environment (no config chagnes)

Dependencies

None

Issue(s) addressed

None, but this provides a correction for documentation about building a new local environment

Checklist

• This PR addresses one issue/problem/enhancement, or has a very good reason for not doing so.
• These changes have been tested on the affected systems and applications.
• All dependency PRs/issues have been resolved and this PR can be merged.

[climbfuji]

Perfect, thank you!