Use plain miniconda in CI

[sfinkens]

Do not use mamba at all. From slack discussion:

I just tested and yeah not using mamba works as well as forcing the mamba version

• [X] Closes #2917

[codecov[bot]]

Codecov Report

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

Project coverage is 96.06%. Comparing base (6a4f2af) to head (9fb59bd). Report is 16 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2919 +/- ## ======================================= Coverage 96.06% 96.06% ======================================= Files 373 373 Lines 54406 54406 ======================================= Hits 52267 52267 Misses 2139 2139 ``` | [Flag](https://app.codecov.io/gh/pytroll/satpy/pull/2919/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | Coverage Δ | | |---|---|---| | [behaviourtests](https://app.codecov.io/gh/pytroll/satpy/pull/2919/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `3.99% <ø> (ø)` | | | [unittests](https://app.codecov.io/gh/pytroll/satpy/pull/2919/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `96.16% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll#carryforward-flags-in-the-pull-request-comment) to find out more.

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[coveralls]

Pull Request Test Coverage Report for Build 11138990341

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.164%

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Totals
Change from base Build 11128129441:	0.0%
Covered Lines:	52501
Relevant Lines:	54595

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💛 - Coveralls