Worker proto use proper serialiser for BuildMode

Do this instead of an unchecked cast

I redid this to use the serialisation framework (including a unit test),
but I am keeping the reference to credit Jade for spotting the issue.

Change-Id: Icf6af7935e8f139bef36b40ad475e973aa48855c
(adapted from commit 2a7a824d83dc5fb33326b8b89625685f283a743b)

Co-Authored-By: Jade Lovelace <lix@jade.fyi>

tests/unit/libstore/worker-protocol.cc

@@ -529,6 +529,17 @@ VERSIONED_CHARACTERIZATION_TEST(
         }),
     }))
 
+VERSIONED_CHARACTERIZATION_TEST(
+    WorkerProtoTest,
+    buildMode,
+    "build-mode",
+    defaultVersion,
+    (std::tuple<BuildMode, BuildMode, BuildMode> {
+        bmNormal,
+        bmRepair,
+        bmCheck,
+    }))
+
 VERSIONED_CHARACTERIZATION_TEST(
     WorkerProtoTest,
     optionalTrustedFlag,