ci: use ignore-error instead of ignore-errors for cache-to

BuildKit's docs say the correct option here is `ignore-error`. I
guess BuildKit doesn't reject unknown arguments?!

.github/workflows/linux.yml

@@ -98,7 +98,7 @@ jobs:
             type=registry,ref=ghcr.io/${{ env.REPO_NAME }}:${{ matrix.image }}-main
             type=registry,ref=ghcr.io/indygreg/python-build-standalone:${{ matrix.image }}-main
           cache-to: |
-            type=registry,ref=ghcr.io/${{ env.REPO_NAME }}:${{ matrix.image }}-${{ env.GIT_REF_NAME }},ignore-errors=true
+            type=registry,ref=ghcr.io/${{ env.REPO_NAME }}:${{ matrix.image }}-${{ env.GIT_REF_NAME }},ignore-error=true
           outputs: |
             type=docker,dest=build/image-${{ matrix.image }}.tar