Support rendering rss on render_site()

[javierluraschi]

Running rmarkdown::render_site() under the https://github.com/rstudio/ai-blog repo does not generate the RSS feed.

This fix searches for the active post being rendered and matches the article$path with the existing local path to the Rmd being generated.

Fix seems to work fine even when using rmarkdown::render_site(input_path = "../ai-blog"), likely due to rmarkdown setting a consistent path while building the site.

[jjallaire]

@javierluraschi I had to revert this as running render_site() on the AI blog with this change applies results in an error (it attempts to render a post which gives an error for data.table not installed and another png related. here). Revert here :6e3da92

We can merge a version of this that doesn't force re-renders of earlier posts but I am just about to wrap up a release for CRAN and would like to see any change we make tested more broadly before it goes into a CRAN release.

[javierluraschi]

Ah yes, there are chunks that do run and require data.table, tracking with rstudio/ai-blog#117.

Sent another PR to opt-in to fully rebuild the RSS with #190 -- Probably worth skipping for CRAN release.