Use default solver (libmamba) when building conda package

[esantorella]

Motivation

This issue should be resolved now that libmamba 2.0.0 is out.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Run nightly cron and make sure conda package build succeeds

[esantorella]

Nightly cron (hoping this works): https://github.com/esantorella/botorch/actions/runs/11151428607

Edit: No, I can't do this on my own fork.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.98%. Comparing base (68faeff) to head (2f9c94e). Report is 4 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2562 +/- ## ========================================== - Coverage 99.98% 99.98% -0.01% ========================================== Files 193 193 Lines 17111 17071 -40 ========================================== - Hits 17108 17068 -40 Misses 3 3 ```

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[facebook-github-bot]

@esantorella has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

@esantorella merged this pull request in pytorch/botorch@df93789c4a583b8b202d4076cb4a29b9071cf96c.