libexec: unlazify launch-builder capnp message reading

StreamFdMessageReader reads message data *lazily*. if you don't access
all segments of the message before you close the underlying stream you
may find yourselv reading from something *very* different, and in this
case that something is The Void™. this causes reads to fail, writes on
the other side to fail to match, and finally our build launch to fail.
this does not lead to happy outcomes, so we will copy the full message
into a fresh new buffer *before* we try to access any of its contents.

fixes #1118

all-analysis-by: deprekated <kate@lix.systems>
Change-Id: I105540831fde855817194e9e539acf177f54a6f4

lix/libexec/launch-builder.cc

@@ -116,9 +116,10 @@ int main(int argc, char * argv[])
     bool sendException = true;
 
     try {
-        capnp::StreamFdMessageReader reader(STDIN_FILENO);
+        capnp::MallocMessageBuilder buf;
+        capnp::readMessageCopyFromFd(STDIN_FILENO, buf);
 
-        auto request = reader.getRoot<build::Request>();
+        auto request = buf.getRoot<build::Request>().asReader();
 
         printDebugLogs = request.getDebug();