Add protection to make sure p_astro plots only generated for files named correctly

This MR just ensures that we're only making p-astro plots for the main p-astro files, i.e. pipeline.p_astro.json where pipeline currently is one of gstlal, mbta, pycbc, or spiir. Previously we just checked that the file ended with p_astro.json, but if pipelines upload multiple p-astros (like gstlal currently does), we could end up making plots for other p-astro variations which we probably don't want.