A few fixes/updates to AppVeyor/micromamba config

[pont-us]

Closes #1050 .

• Use current official/canonical URLs to fetch micromamba binaries (old ones still work at present, but might break in future).

• Fetch with curl, not wget, on Ubuntu. No change in functionality, but this makes it consistent with the Mac config so easier to understand and maintain.

• Add missing --root-prefix parameter to the micromamba shell invocation. Previously we used the synonymous -p parameter for this, but this will be retired in the upcoming micromamba 2.0 release (see https://mamba.readthedocs.io/en/latest/developer_zone/changes-2.0.html#micromamba ).

• Execute micromamba.sh, not mamba.sh, as the profile set-up script (since the latter is not found).

Checklist:

• [ ] Add unit tests and/or doctests in docstrings n/a
• [ ] Add docstrings and API docs for any new/modified user-facing classes and functions n/a
• [ ] New/modified features documented in docs/source/* n/a
• [ ] Changes documented in CHANGES.md n/a
• GitHub CI passes
• AppVeyor CI passes
• Test coverage remains or increases (target 100%)

[konstntokas]

Thank you!

I removed the -p flag and the root prefix turned into an argument but using --root-prefixincreases readability. I did not check for the URL updates. So, cool that looked into it. :)