Docker image tweaks

[danielhollas]

Some tweaks to Docker images

• Don't pin mamba version, I think we always want the latest version (similar to pip). Relatedly, the latest versions of conda actually use the libmamba resolver so there's probably little benefit of using mamba over conda now. But I haven't explored it in detailed yet so I did not change it in this PR.
• set PIP_USER=1, which will make the --user flag to pip default. We want users to install to ~/.local since the home directory is preserved on a volume when the container stops. We do the same in aiidalab-docker-stack
• xz-utils package was being installed twice

[codecov[bot]]

Codecov Report

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

Project coverage is 77.83%. Comparing base (ef60b66) to head (26b9689). Report is 39 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #6466 +/- ## ========================================== + Coverage 77.51% 77.83% +0.33% ========================================== Files 560 561 +1 Lines 41444 41767 +323 ========================================== + Hits 32120 32505 +385 + Misses 9324 9262 -62 ``` | [Flag](https://app.codecov.io/gh/aiidateam/aiida-core/pull/6466/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidateam) | Coverage Δ | | |---|---|---| | [presto](https://app.codecov.io/gh/aiidateam/aiida-core/pull/6466/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidateam) | `73.15% <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=aiidateam#carryforward-flags-in-the-pull-request-comment) to find out more.

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