Don't put artifacts in a build/ subdirectory

When the user says "--output mydir" they really expect the artifacts to
be in `mydir`, not in `mydir/build`.

See also:

- kairos-io/osbuilder#192 (comment)
- kairos-io/kairos-docs#311 (comment)

Signed-off-by: Dimitris Karakasilis <[email protected]>

deployer/steps.go

@@ -186,7 +186,7 @@ func (d *Deployer) tmpRootFs() string {
 }
 
 func (d *Deployer) destination() string {
-	return d.Config.StateDir("build")
+	return d.Config.State
 }
 
 func (d *Deployer) isoFile() string {