[CI] Replace all forward slashes in artifact index (#1316)

Not just the first one.

Author: giordano

.github/workflows/test-gb-25.yml

@@ -67,7 +67,7 @@ jobs:
           # Artifact names cannot include forward slashes and some other characters, here we
           # do some sanitisation.
           ARTIFACT_INDEX=${{ matrix.julia_version }}-${{ matrix.xla_commit }}-${{ matrix.gb25_commit }}-${{ matrix.reactant_commit }}
-          echo "ARTIFACT_INDEX=${ARTIFACT_INDEX/\//_}" >> "${GITHUB_ENV}"
+          echo "ARTIFACT_INDEX=${ARTIFACT_INDEX//\//_}" >> "${GITHUB_ENV}"
       - uses: bazel-contrib/[email protected]
         name: Set up Bazel
         with: