Add "Externally Managed Environments" support to block global package installations via pip

Description:
To prevent system package breakage, many Linux distributions are now restricting pip install to virtual environments or uv environments (something private either way). In self-hosted environments, especially when using ephemeral runners, the hosted tools cache functionality is critical for scaling.

The current methodology allows a pip install to install directly to the cached directory for a given Python after installation, which leads to potentially broken dependency trees and all other kinds of errors if developers are not careful with their package installs. It can also lead to weird race conditions in the same way.

I propose a new input for actions/setup-python that creates the EXTERNALLY-MANAGED file right before a version of Python is marked as available (e.g. creation of the x64.complete file) that would block global pip installs.

https://packaging.python.org/en/latest/specifications/externally-managed-environments/#externally-managed-environments

Justification:
Did all of this in the above. Didn't read the template first.

Are you willing to submit a PR?
Absolutely. This is more of a RFC to gauge the level of interest here.

Inizia leggendo il percorso di setup-python tool-cache che contrassegna una versione di Python installata come disponibile, quindi esamina la specifica collegata sugli ambienti gestiti esternamente. Determina come debba essere creato il marker prima che la disponibilità venga registrata e quali test esistenti coprano le installazioni di Python memorizzate nella cache. Il lavoro è completato quando le installazioni globali di pip sono bloccate per l’interprete memorizzato nella cache senza interrompere i workflow supportati di virtual-environment o uv.