Sys.which() is enough from a terminal and often not enough anywhere else. An
app launched from RStudio or any GUI inherits a minimal environment rather
than the shell's, so a client installed under conda is on the user's PATH and
not on R's. The usual install locations are checked as well, and the answer is
an absolute path so that nothing downstream has to search again — including
the parallel workers, which start with no PATH additions at all.