Disable cosign signing

Since the current builder contains a non-working cosign version, we have
to disable signing the builder as well. This will lead to a build which
is unsigned, but will allow us to build another signed builder, and
finally reenable signature checking as well.

.github/workflows/publish.yml

@@ -57,5 +57,4 @@ jobs:
           args: |
             --${{ matrix.architecture }} \
             --target /data \
-            --cosign \
             --generic $GIT_TAG_NAME