Add --env-var to bun run

[feledori]

Currently you can tell bun to load an .env.* file with --env-file which is really nice, but sometime you just want to pass a single env var to a command to change the behaviour. The guide for env vars tells you to pass a single env var before the command. This works well on unix based systems, but for windows you need to install a separate package like cross-env to pass an env var to a command. Would be a nice to have if this can be build into bun run.