Fix conda workflow, new badges

[jhdark]

Proposed changes

With the previous issues with conda, the workflow was disabled as it was inconsistent, often failing on setup of the environment. A small change is provided which changes how mamba is defined in setting up the environment.

Additionally, badges of the workflow results are now shown on the README.md

Types of changes

What types of changes does your code introduce to FESTIM?

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code refactoring
• Documentation Update (if none of the other choices apply)
• New tests

Checklist

• Black formatted
• Unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (4836eba) 95.83% compared to head (2249601) 95.88%.
Report is 11 commits behind head on fenicsx.

❗ Current head 2249601 differs from pull request most recent head 43d5c18. Consider uploading reports for the commit 43d5c18 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           fenicsx     #610      +/-   ##
===========================================
+ Coverage    95.83%   95.88%   +0.04%     
===========================================
  Files           10       10              
  Lines          264      267       +3     
===========================================
+ Hits           253      256       +3     
  Misses          11       11              

see 2 files with indirect coverage changes

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