Start user shells in proper PAM sessions

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**Is your feature request related to a problem? Please describe.**
In NixOS, we rely on PAM to set up some important environment variables, including some related to locales, via `pam_env.so`. Right now WSL starts a single non-interactive shell with `login -f` to trigger the user systemd instance, but all the interactive shells are spawned bypassing PAM, and therefore not importing the environment variables.

**Describe the solution you'd like**
WSL starts every interactive shell instance via `login` or a similar mechanism that runs PAM modules.

**Describe alternatives you've considered**
Right now we're considering wrapping the user-selected shell in a script that updates the environment manually, but this will not be correct if any other PAM modules are involved.

**Additional context**
The specific issue that led me to notice this was zsh failing to discover locales when spawned through WSL, as we require the LOCALE_ARCHIVE environment variable to be set for locale archives to be discovered.

Start by tracing WSL's interactive-shell launch path and compare it with the existing non-interactive `login -f` path. Check how PAM sessions, including `pam_env.so`, are invoked before launching the user-selected shell, and verify that locale variables such as `LOCALE_ARCHIVE` are available in zsh.

Written by the indexing model from the issue text.