Removing linux/arm/v7 from nightly build

I have no idea why this platform is causing build error
especially stub file generation. For some reason, it can't
find the src/*.stub filed.

Author: ktakashi

.github/workflows/nightly-build.yml

@@ -53,7 +53,7 @@ jobs:
         with:
           file: .ci/Dockerfile.nightly
           push: true
-          platforms: linux/amd64,linux/arm64/v8,linux/arm/v7,linux/ppc64le
+          platforms: linux/amd64,linux/arm64/v8,linux/ppc64le
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
           build-args: |