Keep inst/doc and continue when answering No to the clean_doc prompt

[taekop]

Answering "No" to the interactive "Building the package will delete inst/doc, are you sure?" prompt hits a bare return() in build_setup_source(). That returns NULL to build(), which then fails with an unrelated error:

build(clean_doc = NULL)  # interactive, inst/doc exists, answer 2
#> Error: no("wd") || is_string(wd) is not TRUE

Now "No" (or cancelling the prompt) skips the deletion and the build continues, so inst/doc is kept, matching what clean_doc = FALSE does. Only "Yes" deletes it. The prompt now reads "Delete it and build ('Yes'), or keep it and build ('No')?" so it's clear what each answer does.

The test mocks the prompt through a small is_interactive() wrapper, because mocking base interactive() doesn't take effect once the function has been byte-compiled.

Before/after on 19 build() setups (clean_doc NULL/TRUE/FALSE, Config/build/clean-inst-doc TRUE/FALSE/unset, vignettes on/off, no inst/doc, binary build, interactive Yes/No): the other 18 give identical tarball contents. The one that changes is interactive "No" with nothing configured, which used to error. Full test suite passes.

Closes #186

[gaborcsardi]

Thanks!