[GHA] Explicitly install Julia for whitespace workflow (#56468)

So far we relied on the fact that Julia comes in the default Ubuntu
images on GitHub Actions runners, but this may change in the future
(although there's apparently no plan in this direction for the time
being). To make the workflow more future-proof, we now explicitly
install Julia using a dedicated workflow.

.github/workflows/Whitespace.yml

@@ -18,6 +18,9 @@ jobs:
         uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
         with:
           persist-credentials: false
+      - uses: julia-actions/setup-julia@9b79636afcfb07ab02c256cede01fe2db6ba808c # v2.6.0
+        with:
+          version: '1'
       - name: Check whitespace
         run: |
           contrib/check-whitespace.jl