ruby: detect rbenv shims for other versions (#8733)

When rbenv is used for managing Ruby installations, just checking for an
executable called "neovim-ruby-host" is not enough. It has to be run as well.

If it does not return 0, then neovim-ruby-host is merely a shim for another Ruby
installation.

  $ rbenv versions
    2.5.0
  * 2.5.1 (set by /Users/mhi/.rbenv/version)

  $ rbenv whence neovim-ruby-host
  2.5.0

  $ which neovim-ruby-host
  /Users/mhi/.rbenv/shims/neovim-ruby-host

  $ neovim-ruby-host
  rbenv: neovim-ruby-host: command not found

  The `neovim-ruby-host' command exists in these Ruby versions:
    2.5.0

  $ echo $?
  127

Additionally, the detection logic was moved from provider#ruby#Detect() to
s:detect(), because the former is run in the sandbox which forbids calling
system().

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