[linter] Runtime preload should only be required if boilerplate is still present

[schlessera]

In #1248, we've skipped adding the AMP runtime preload(s) when the boilerplate had already been removed.

However, the linter still requires the runtime preload to be present unconditionally in https://github.com/ampproject/amp-toolbox/blob/main/packages/linter/src/rules/RuntimeIsPreloaded.ts.

The linter should also check for the presence of the boilerplate and only require the runtime preload if the boilerplate is still present.

[westonruter]

This issue keeps getting reported by users. We should update Pixi sooner rather than later.