stages/dnf: add mandatory basearch argument

We've been effectively using the basearch of the host, making the stage
non-reproducible: if the same pipeline was run on machines with
different architectures, it would produce different results. However,
pipelines producing different outputs must be different. Thus, this
patch includes the basearch in the pipeline.

In principle, this allows cross-arch builds. dnf should be the only
stage running binaries from the target tree. This is not yet tested.

samples/base.json

@@ -5,6 +5,7 @@
       "name": "org.osbuild.dnf",
       "options": {
         "releasever": "30",
+        "basearch": "x86_64",
         "repos": {
           "fedora": {
             "name": "Fedora",