fix(ci): pass --all to mix igniter.upgrade so the job stops failing

[jimsynz]

The shared mix igniter.upgrade job fails on every dependabot PR across the
ecosystem:

##[group]Run mix igniter.upgrade --git-ci --yes
Must specify at least one package to upgrade or use --all to upgrade all packages.
##[error]Process completed with exit code 1.

The job passes neither package names nor --all, so mix igniter.upgrade
rejects the invocation before doing any work. Confirmed identical in ash,
ash_postgres, ash_phoenix, ash_graphql and ash_ai — anything using this
workflow.

It broke in ff418c2 ("ci: setup action properly for igniter.upgrade"), which
converted the step from a run: to the staple-actions/mix-task action and
dropped the argument along the way:

-        run: mix igniter.upgrade ${{steps.dependabot-metadata.outputs.dependency-names}} --git-ci
+          task: igniter.upgrade --git-ci

Nothing surfaced it because the step only runs for dependabot PRs. On tag pushes
it's skipped, and build-release/hex_publish gate on !failure(), which
treats skipped as fine — so releases were never blocked and the failure just sat
on dependabot PRs as a red X.

--all is the right argument rather than restoring dependency-names: under
--git-ci the upgrade set is derived entirely from diffing HEAD~1:mix.lock
against the loaded deps, and the dependency fetch is skipped, so --all only
satisfies the argument check. It doesn't widen what gets upgraded or touch
mix.lock.

Verified end-to-end in a scratch project with a simulated dependabot lock bump:
--git-ci --yes exits 1 with the error above, --git-ci --yes --all exits 0 and
reports jason 1.4.4 => 1.4.5 with mix.lock unmodified.

Igniter shouldn't have demanded the argument in the first place, since it never
reads it on the --git-ci path — ash-project/igniter#397 fixes that end. This
change is worth landing regardless: it's a one-liner, it works against released
igniter, and it unblocks dependabot PRs now instead of after an igniter release.

[zachdaniel]

🚀 Thank you for your contribution! 🚀