Prepare for new menuinst; non-win platforms

[jaimergp]

Part of the CEP efforts.

Right now this is using menuinst from the experimental builds available at jaimergp/label/menuinst_dev, which are coming from this other PR (cep branch).

Summary of changes:

• Remove on_win checks for menuinst code paths.
• Use the new menuinst API, falling back to menuinst._legacy when needed.
• Enable --shortcuts and --no-shortcuts CLI flags in non-windows platforms.
• Add --shortcuts-only†‡ flag to CLI and context, which adds better support for constructor's menu_packages option.
• Add menuinst=2 specific tests in the integration suite

---

† This allows us to remove the conda-standalone constructor --mk-menus/--rm-menus code paths, simplifying the patches on the conda-standalone project. Note this can be further simplified by improving the conda run support on conda-standalone.

‡ This can be implemented with a dual --shortcuts too. If shortcuts is set to True (bare --shortcuts), same behaviour is kept. If the user sets it to a list of packages, then it behaves as shortcuts_only (e.g. --shortcuts=package_a,package_b). However, this is trickier to implement (and I am not sure it's even possible without big changes to context / argparse). Hence why I opted for --shortcuts-only above: it is one more flag, yes, but maintains better legacy behaviour and we can see precedents for this in the --channel-priority expansion. Note it can be used several times like --shortcuts-only=python --shortcuts-only=numpy.

[jaimergp]

@kenodegard, I am seeing some environment stacking errors strikingly similar those described in #11257 (comment) and related work (linux integration group 1). I don't get how or why changing menuinst stuff would affect that part of the code but what do I know... do you mind sharing any pointers you might have if you have time at some point? Thank you!

[jaimergp]

Well turns out you should NOT install things after conda init --dev because if the dependencies pull in conda again it might undo the initialization and you are back to store-bought conda, which might break tests unexpectedly. I am surprised it didn't break more stuff, but I guess we should add a test to check this condition specifically...

[jaimergp]

Continues in #11882 (token needed for canary builds)