fix: Adapt shell init for mamba 2 #1846

Job	Run time
ubuntu-latest, Python 3.9, conda-standalone	10m 7s
ubuntu-latest, Python 3.10, conda-standalone-nightly	9m 13s
ubuntu-latest, Python 3.11, micromamba	8m 1s
ubuntu-latest, Python 3.12, conda-standalone	8m 55s
macos-13, Python 3.9, conda-standalone-nightly	36m 25s
macos-latest, Python 3.11, conda-standalone	18m 52s
macos-latest, Python 3.12, micromamba	13m 30s
windows-latest, Python 3.9, conda-standalone-nightly	29m 43s
windows-latest, Python 3.10, conda-standalone	29m 52s
windows-latest, Python 3.11, conda-standalone-nightly	29m 7s
windows-latest, Python 3.12, conda-standalone	30m 50s
Report failures	0s
Canary Build	0s
	3h 44m 35s

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