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

elixir-ci-reusable.yml:105 had the same bare-`if:`-at-job-level pattern
that #334 fixed in rust-ci-reusable.yml. Surfaced via the sweep recipe
documented in standards#334:

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

The mix.exs guard is structurally identical to the Cargo.toml guard
that broke 43+ Rust callers — same fix.

Follow-up audit: same grep found ZERO additional hits in other
reusables (governance/secret-scanner/scorecard/mirror/casket/dogfood).

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: