[ci] update to r-lib/actions v2.12.0, align 'pip' and 'python' to same interpreter in 'build-python.sh'

[jameslamb]

Fixes #7248

See the linked issues there. I think the latest r-lib/actions/setup-tinytex needs to be used because something has been changed about https://tinytex.yihui.org/install-bin-windows.ps1 (it appears to be there, but the action fails to download it).

Fixes #7258

• directly declares pip dependency in conda environments
• replaces pip w/ python -m pip in build-python.sh, to ensure all processes use the same Python interpreter

Closes #7257

• this will not be mergeable until the issues with Ubuntu package servers are resolved

[jameslamb]

The current CI failures like this:

Processing triggers for dbus (1.12.20-2ubuntu4.1) ...
Traceback (most recent call last):
  File "/usr/bin/add-apt-repository", line 364, in <module>
    sys.exit(0 if addaptrepo.main() else 1)
  File "/usr/bin/add-apt-repository", line 347, in main
    shortcut = handler(source, **shortcut_params)
  File "/usr/lib/python3/dist-packages/softwareproperties/shortcuts.py", line 40, in shortcut_handler
    return handler(shortcut, **kwargs)
  File "/usr/lib/python3/dist-packages/softwareproperties/ppa.py", line 82, in __init__
    if self.lpppa.publish_debug_symbols:
  File "/usr/lib/python3/dist-packages/softwareproperties/ppa.py", line 120, in lpppa
    self._lpppa = self.lpteam.getPPAByName(name=self.ppaname)
  File "/usr/lib/python3/dist-packages/lazr/restfulclient/resource.py", line 592, in __call__
    response, content = self.root._browser._request(
  File "/usr/lib/python3/dist-packages/lazr/restfulclient/_browser.py", line 429, in _request
    raise error
lazr.restfulclient.errors.ServerError: HTTP Error 504: Gateway Time-out

Are probably due to the ongoing Ubuntu / Canonical outage that's been going on most of today: https://status.canonical.com/#/incident/KNms6QK9ewuzz-7xUsPsNylV20jEt5kyKsd8A-3ptQEHpOd8VQ40ZQs-KD81fboQXeGZB94okNHdHBGlCv58Sw==

Nothing we can do with those but wait for them to be resolved.

[jameslamb]

yikes, things have not improved for Ubuntu package servers. They've fully turned off status updates and monitoring data at https://status.canonical.com/, with the note:

_Canonical’s web infrastructure is under a sustained, cross-border attack and we are working to address it. We will provide more information in our official channels as soon as we are able to. _

[jameslamb]

It seems the ongoing Ubuntu outage is still blocking us. Several Ubuntu CI jobs are still failing with timeouts during downloads 🙃

[jameslamb]

More CI issues have come up in the time this has been open 😭

So this is turning into another one of those "fix all the CI issues" PRs like #7236. I've added tracked issues for those new problems and updated the title and description.