devel/RStudio: Fix bug that RStudio-server didn't have /usr/local/bin in PATH

This was previously fixed in post-install but the previous fix
relied on the other path that changed, which eroded this fix.

The new fix adds /usr/local/bin to PATH in the C++ code.

(cherry picked from commit 9884a43067a47d9f4088306ad29d1ec373b25eee)