fix(elixir-ci-reusable): wrap bare job-level if: in ${{ }} (same anti-pattern as #322/#334) (#336)

## Summary

Same root cause as #322 / #334 — \`elixir-ci-reusable.yml:105\` had a
bare \`if:\` at job level containing \`hashFiles(format('{0}/mix.exs',
inputs.working_directory))\`. Surfaced via the sweep recipe from #334:

\`\`\`bash
grep -nE '^\\s+if: [^\$]+(hashFiles|format|inputs\\.)'
.github/workflows/*-reusable.yml
\`\`\`

Only hit — every other reusable is clean. Same 2-line `\${{ }}` wrap.

## Test plan

- [x] Sweep recipe returns no remaining hits across reusables
- [x] yamllint clean
- [x] Pattern matches #334's exact diff shape

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

.github/workflows/elixir-ci-reusable.yml

@@ -102,7 +102,7 @@ jobs:
     name: Compile + test
     runs-on: ${{ inputs.runs-on }}
     # Guard on mix.exs so the wrapper is safe to add unconditionally.
-    if: hashFiles(format('{0}/mix.exs', inputs.working_directory)) != ''
+    if: ${{ hashFiles(format('{0}/mix.exs', inputs.working_directory)) != '' }}
     permissions:
       contents: read
     env: