examples: support .env files during vite build (#4866)

## Motivation

In #4578 we added a
check to make sure required environment variables are available for the
build. However, during the check, we read variables directly from the
process environment, which doesn't contain any values specified in the
`.env.*` files.

## Proposal

Use Vite's `loadEnv` function to get additional variables from `.env`
files, re-enabling the use of `.env` files to store build variables.

## Test Plan

Tested locally.

## Release Plan

- Nothing to do / These changes follow the usual release cycle.

## Links

- [reviewer
checklist](https://github.com/linera-io/linera-protocol/blob/main/CONTRIBUTING.md#reviewer-checklist)

Author: Twey