mkosi: Only make build sources ephemeral if NO_BUILD is not enabled

If we're not building distribution packages from source, there's no
need to make the build sources ephemeral so don't enable the setting
if NO_BUILD is enabled.

Author: DaanDeMeyer

mkosi.conf

@@ -33,7 +33,6 @@ OutputDirectory=build/mkosi.output
 ToolsTree=default
 BuildDirectory=build/mkosi.builddir
 CacheDirectory=build/mkosi.cache
-BuildSourcesEphemeral=yes
 Incremental=yes
 
 [Validation]

mkosi.conf.d/20-build.conf

@@ -1,9 +1,10 @@
 # SPDX-License-Identifier: LGPL-2.1-or-later
 
-# Add a dependency on the build image unless NO_BUILD=1.
+# Add a dependency on the build image and make the build sources ephemeral unless NO_BUILD=1.
 
 [Match]
 Environment=!NO_BUILD=1
 
 [Config]
 Dependencies=build
+BuildSourcesEphemeral=yes