_context.py: Put the source_cache_specs close to the other cache specs

Instead of mixing it up with the directories
diff --git a/src/buildstream/_context.py b/src/buildstream/_context.py
index 19e0781..c0e92b9 100644
--- a/src/buildstream/_context.py
+++ b/src/buildstream/_context.py
@@ -63,9 +63,6 @@
         # The directory where various sources are stored
         self.sourcedir = None
 
-        # specs for source cache remotes
-        self.source_cache_specs = None
-
         # The directory where build sandboxes will be created
         self.builddir = None
 
@@ -85,6 +82,9 @@
         # Default root location for workspaces
         self.workspacedir = None
 
+        # specs for source cache remotes
+        self.source_cache_specs = None
+
         # The locations from which to push and pull prebuilt artifacts
         self.artifact_cache_specs = None