main: tweak handling of --output-name to avoid adding double extensions

This commit tweaks the handling of the `--output-name` option so
that is a name with the same extension as the image is passed that
is just silently ignored. Its a common issue that first time
users run:
```console
$ image-builder build --output-name foo.qcow2 qcow2
```
which currently leads to a foo.qcow2.qcow2. With this commit
the expected "foo.qcow2" will appear.

cmd/image-builder/export_test.go

@@ -16,6 +16,7 @@ var (
 	FindDistro      = findDistro
 	DescribeImage   = describeImage
 	ProgressFromCmd = progressFromCmd
+	BasenameFor     = basenameFor
 )
 
 func MockOsArgs(new []string) (restore func()) {