fix(ci): Proper type of var for platforms in docker build action (#2924)

## What ❔

Proper type of var for platforms in docker build action

## Why ❔

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## Checklist

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- [ ] Tests for the changes have been added / updated.
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.github/workflows/build-runtime-base.yml

@@ -56,9 +56,7 @@ jobs:
         with:
           push: true
           context: .
-          platforms: |
-            - arm64
-            - amd64
+          platforms: arm64, amd64
           file: docker/${{ matrix.name }}/Dockerfile
           labels: |
             org.opencontainers.image.source=https://github.com/matter-labs/zksync-era