remote exceptions: simplify sysinfo processing

in the sysinfo function, there is a way to suppress
all sysinfo output via an env var and just return an
empty string.

so we can expect it is always in unpacked, but it
might be the empty string.

src/borg/remote.py

@@ -209,6 +209,7 @@ def serve(self):
                         else:
                             logging.debug("\n".join(ex_full))
 
+                        sys_info = sysinfo()
                         try:
                             msg = msgpack.packb(
                                 {
@@ -218,7 +219,7 @@ def serve(self):
                                     "exception_full": ex_full,
                                     "exception_short": ex_short,
                                     "exception_trace": ex_trace,
-                                    "sysinfo": sysinfo(),
+                                    "sysinfo": sys_info,
                                 }
                             )
                         except TypeError:
@@ -230,7 +231,7 @@ def serve(self):
                                     "exception_full": ex_full,
                                     "exception_short": ex_short,
                                     "exception_trace": ex_trace,
-                                    "sysinfo": sysinfo(),
+                                    "sysinfo": sys_info,
                                 }
                             )
 
@@ -438,7 +439,7 @@ def exception_full(self):
 
         @property
         def sysinfo(self):
-            return self.unpacked.get("sysinfo", "")
+            return self.unpacked["sysinfo"]
 
     class RPCServerOutdated(Error):
         """Borg server is too old for {}. Required version {}"""