[OMBU-821] Name .tool-versions through setup-ruby's real input

[JuanVqz]

Follow-up to #21, which merged before this fix was pushed.

Why

ruby/setup-ruby@v1 has no ruby-version-file input, so it warned on every run:

Unexpected input(s) 'ruby-version-file', valid inputs are ['ruby-version',
'rubygems', 'bundler', 'bundler-cache', 'working-directory', 'cache-version',
'self-hosted', 'windows-toolchain', 'token']

That name comes from actions/setup-python and setup-node, not from this
action. It is absent from v1, from the latest release v1.321.0, and from
master, so no version bump or pin fixes it.

The input was ignored, and the Ruby version still resolved, because with
ruby-version unset the action tries .ruby-version, then .tool-versions,
then mise.toml:

ruby-version: default
Using 4.0.6 as input from file .tool-versions
ruby 4.0.6 (2026-07-14 revision 03b6d3f889) +PRISM [x86_64-linux]

What changed

Rather than lean on that implicit lookup order, name the file through the input
that does exist. ruby-version accepts .ruby-version, .tool-versions and
mise.toml as literal values, per the action's README.

- uses: ruby/setup-ruby@v1
  with:
    ruby-version: .tool-versions
    bundler-cache: true

Note for skunk.fyi

skunk.fyi's workflow uses the same invalid ruby-version-file input and carries
the same warning. Worth the same fix there.