Refactor create-fuels logic to not depend on process.env.VITEST

[nedsalk]

We are currently using process.env.VITEST in production code to govern whether checks for the existence of fuelup on the user's machine will be done. This environment variable is only defined in our tests. We should remove this in favor of something else, e.g. a flag like pnpm create fuels --skip-fuelup-check. If we were to switch to e.g. bun for testing, the create-fuels tests would fail because the VITEST environment variable isn't defined.