Default RBENV_VERSION to the globally-specified Ruby

Fixes `rbenv install` in a directory with a .ruby-version file that specifies ree

bin/rbenv-install

@@ -142,6 +142,12 @@ if [ -z "${RUBY_BUILD_CACHE_PATH}" ] && [ -d "${RBENV_ROOT}/cache" ]; then
   export RUBY_BUILD_CACHE_PATH="${RBENV_ROOT}/cache"
 fi
 
+# Default RBENV_VERSION to the globally-specified Ruby version. (The
+# REE installer requires an existing Ruby installation to run. An
+# unsatisfied local .ruby-version file can cause the installer to
+# fail.)
+export RBENV_VERSION="$(rbenv global 2>/dev/null || true)"
+
 
 # Execute `before_install` hooks.
 for hook in "${before_hooks[@]}"; do eval "$hook"; done