cmd-build-with-buildah: limit adding overrides mount

If there are no rpms then no point in adding the mount.
Should prevent cosmetic warts like:

```
Updating and loading repositories:
 overrides                              ???% |   0.0   B/s |  -1.0   B |  ?
>>> Curl error (37): Could not read a file:// file for file:///run/src/overrides
>>> Usable URL not found
```

Author: dustymabe

src/cmd-build-with-buildah

@@ -165,8 +165,8 @@ build_with_buildah() {
     if [ -d overrides ]; then
         if [[ -n $(ls overrides/rpm/*.rpm 2> /dev/null) ]]; then
             (cd overrides/rpm && rm -rf .repodata && createrepo_c .)
+            set -- "$@" -v "$(realpath overrides)":/run/src/overrides
         fi
-        set -- "$@" -v "$(realpath overrides)":/run/src/overrides
     fi
 
     if [ -n "$DIRECT" ]; then