Allow `use_virtualenv()` to fall back to looking for system wide installations.

[t-kalinowski]

For example

use_virtualenv("r-keras")

Would look for:

• ~/.virtualenvs/r-keras
• /opt/virtualenvs/r-keras

By default assembled from:

c(Sys.getenv("WORKON_HOME", "~/.virtualenvs/"),
  Sys.getenv("RETICULATE_VIRTUALENV_SEARCH_PATH", "/opt/virtualenvs/"))