gdb/testsuite: fix timeout in server-pipe.exp test

I noticed that the gdb.server/server-pipe.exp test would sometimes
timeout when my machine was more heavily loaded.  Turns out the test
is reading all the shared libraries over GDB's remote protocol, which
can be slow.

We avoid this in other tests by setting the sysroot in GDBFLAGS,
something which is missing from the gdb.server/server-pipe.exp test.

Fix the timeouts by setting sysroot in GDBFLAGS, after this the shared
libraries are no longer copied over the remote protocol, and I no
longer see the test timeout.

gdb/testsuite/gdb.server/server-pipe.exp

@@ -74,7 +74,11 @@ proc do_test { target } {
     }
 }
 
+save_vars { GDBFLAGS } {
+    set GDBFLAGS "$GDBFLAGS -ex \"set sysroot\""
+
     # Test with the two remote protocol types.
     foreach_with_prefix target { remote extended-remote } {
 	do_test ${target}
     }
+}