Improve hint on pip install --system when externally managed

[zanieb]

I had some qualms with #16318

1. The snapshot was specific to uv's managed interpreter due to inclusion of the externally-managed output. This will break downstream distros. We either need to filter the message, or, as done here, install a managed interpreter.
2. It had a custom filter for the interpreter path, which we shouldn't need. If needed, we should fix that in the test context.
3. We already had an implicit hint to create a virtual environment. The change in styling of the new hint hint following it is confusing. It's also confusing to hint creating a virtual environment when --system was used.
4. There were unresolved requested changes to the language for the message / it didn't fit stylistic with our existing ones.
5. The message was also very confusing for a uv-managed interpreter, which is both a "system" Python interpreter (in that it's global) and the opposite of a "system" interpreter per UV_PYTHON_PREFERENCE.

Also problematic, but not addressed here, is that there are other commands that display an externally-managed message, e.g., uv pip sync, but #16318 was just limited to pip install. We should not just implement this in one place — I'll open a tracking issue to consolidate and reuse the logic.

[zanieb]

cc @cmnemoi maybe a few things to learn from here