builder: consider all execution paths

The builds step implicitly depends on the execution path of all
dependencies. So far we only tracked arguments. But some build systems
also store the absolute path to tools. In this case, the build will fail
if

pym/bob/builder.py

@@ -1295,7 +1295,7 @@ async def _cookBuildStep(self, buildStep, depth, buildBuildId):
         # dependency directories change.
         buildVariantId = await self.__getIncrementalVariantId(buildStep)
         buildDigest = [buildVariantId, buildStep.getExecPath()] + \
-            [ i.getExecPath(buildStep) for i in buildStep.getArguments() if i.isValid() ]
+            [ i.getExecPath(buildStep) for i in buildStep.getAllDepSteps() if i.isValid() ]
 
         # get directory into shape
         (prettyBuildPath, created) = self._constructDir(buildStep, "build")