Enh: Start from clone of python conda environment for speedup

[klieret]

Saves about 20s (out of 30s for setting up the conda env to begin with).

Currently only enabled for what's like the single issues (as opposed to the SWE-bench ones).

Todo:

• [x] Does this actually work with 3.9?
• [x] Test coverage will be broken until the new docker image is available after this PR is merged
• [x] Roll back setup.sh

Closes #491

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 68.75000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 76.17%. Comparing base (d9d9a9b) to head (d96c0d6). Report is 3 commits behind head on main.

Files	Patch %	Lines
sweagent/environment/swe_env.py	37.50%	5 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #548 +/- ## ======================================= Coverage 76.16% 76.17% ======================================= Files 20 20 Lines 3239 3261 +22 ======================================= + Hits 2467 2484 +17 - Misses 772 777 +5 ```

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