Avoid rbenv-exec calling out to rbenv-version-name twice

Running any shim (and thus `rbenv-exec`) would always execute
`rbenv-version-name` twice: once in `rbenv-exec` and another time in
`rbenv-which`, even though RBENV_VERSION variable would have already
been populated at this point.

Now RBENV_VERSION is respected within `rbenv-which`.

test/which.bats

@@ -72,3 +72,12 @@ SH
   assert_success
   assert_output "HELLO=:hello:ugly:world:again"
 }
+
+@test "discovers version from rbenv-version-name" {
+  mkdir -p "$RBENV_ROOT"
+  cat > "${RBENV_ROOT}/version" <<<"1.8"
+  create_executable "1.8" "ruby"
+
+  RBENV_VERSION= run rbenv-which ruby
+  assert_success "${RBENV_ROOT}/versions/1.8/bin/ruby"
+}