Change path for dnf arch override so basearch is not overwritten.

After writing the basearch value to /etc/dnf/vars/basearch the
arch value was overwriting the same file.  This appears to be
incorrect, so changing it to write /etc/dnf/vars/arch, which
matches the subsequent 'yum' code paths.

Change-Id: I5da54f03224c11f9e286f16b68533936c4174c2a

elements/rpm-distro/pre-install.d/01-override-yum-arch

@@ -28,7 +28,7 @@ fi
 if [[ $DISTRO_NAME == "fedora" && $DIB_RELEASE -ge 22 ]]; then
     mkdir -p /etc/dnf/vars
     echo $basearch > /etc/dnf/vars/basearch
-    echo $arch > /etc/dnf/vars/basearch
+    echo $arch > /etc/dnf/vars/arch
 else
     echo $basearch > /etc/yum/vars/basearch
     echo $arch > /etc/yum/vars/arch