pin mamba in conda lock creation github action

[valeriupredoi]

Description

Related to https://github.com/ESMValGroup/ESMValTool/issues/3782 - currently conda-lock is struggling with mamba>=2 but Ben Mares at conda-lock has fixed the issue, and all we need to do is wait for a major release of conda-lock; in the meantime the workaround is to pin mamba in the Github action, so we can get our lock file generated.

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[codecov[bot]]

Codecov Report

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

Project coverage is 94.77%. Comparing base (f622e40) to head (16c762a). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2561 +/- ## ======================================= Coverage 94.77% 94.77% ======================================= Files 251 251 Lines 14266 14266 ======================================= Hits 13520 13520 Misses 746 746 ```

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[valeriupredoi]

cheers @bettina-gier - good point, the issue I am pointing to here is actually a more up to date one, for Tool I'd have to open yet another PR to get that in in a comment, and by now, I reckon you've had enough of these reviews, so I don't want to scare off our new techie bod :grin:

[valeriupredoi]

but - excellent review BTW - this is what we want - reviewers to pay attention to all the details, not like me - approve a Draft PR :rofl:

[bettina-gier]

if you're just changing the link in the tool I can approve it without having to think, so all good. Better to be consistent than to save those 5min

[valeriupredoi]

if you're just changing the link in the tool I can approve it without having to think, so all good. Better to be consistent than to save those 5min

sounds good - let me do that after I merge this :smiley: