build: use build_by_default in cargo-build target

build_always_stale is deprecated and build_by_default can be
used now that we don't have build.rs.

Requires that we list our sources.

podcasts-downloader/src/meson.build

@@ -0,0 +1,5 @@
+downloader_sources = files(
+  'downloader.rs',
+  'errors.rs',
+  'lib.rs'
+)