feat(fleet): add optional cascade_version inputs to dispatch-suite action

[joshua-temple]

Problem

A future fleet lane needs to forward the rc version under test (tag and
peeled commit sha) into a downstream example-repo suite's
workflow_dispatch. The dispatch-suite composite action has no place to
carry those values today.

Fix

Add two optional inputs to .github/actions/dispatch-suite/action.yaml,
cascade_version and cascade_version_sha, both empty by default. They
are plumbed into the step's env block and forwarded as -f args to
gh workflow run only when non-empty. No caller sets them yet and no
example-repo suite declares the inputs, so this is inert: every existing
fleet lane produces the exact same gh workflow run invocation as before.

Passing only when non-empty is load bearing: gh workflow run errors with
"unexpected inputs" if you pass an undeclared input, so an unconditional
pass would break every current suite.

Verification

• shellcheck on the extracted embedded script: clean
• go build ./... && go test ./... && golangci-lint run ./... at repo
  root: clean, unaffected
• Manually confirmed the generated gh workflow run command is
  byte-for-byte identical to before when both new inputs are empty
  (the default)

Note: actionlint cannot parse composite action.yaml files as
workflows (it expects jobs/on); this is a pre-existing limitation
that also reproduces on the unmodified setup-cli/action.yaml, not a
regression from this change.