[gdb/testsuite] Set sysroot earlier in local-board.exp

When running test-case gdb.base/batch-exit-status.exp for native, it passes.
But with target board cc-with-debug-names, we run into (added missing double
quotes for clarity):
...
builtin_spawn $build/gdb/testsuite/../../gdb/gdb -nw -nx \
  -data-directory $build/gdb/testsuite/../data-directory \
  -iex "set height 0" -iex "set width 0" -ex "set sysroot" -batch ""^M
: No such file or directory.^M
PASS: gdb.base/batch-exit-status.exp: \
  : No such file or directory\.: [lindex $result 2] == 0
FAIL: gdb.base/batch-exit-status.exp: \
  : No such file or directory\.: [lindex $result 3] == $expect_status
...

The difference between the passing and failing case is that with native we
have (leaving out set height/width for brevity):
...
$ gdb -batch ""; echo $?
: No such file or directory.
1
...
and with target board cc-with-debug-names:
...
$ gdb -ex "set sysroot" -batch ""; echo $?
: No such file or directory.
0
...

The difference is expected.  GDB returns the exit status of the last executed
command.  In the former case that's 'file ""', which fails.  In the latter case,
that's 'set sysroot', which succeeds.

Fix this by setting sysroot using -iex instead of -ex in local-board.exp, such
that we have the expected:
...
$ gdb -iex "set sysroot" -batch ""; echo $?
: No such file or directory.
1
...

Tested on x86_64-linux.

gdb/testsuite/boards/local-board.exp

@@ -24,4 +24,4 @@ set baseboard [lindex [split $board "/"] 0]
 set board_info($baseboard,isremote) 0
 
 # Set sysroot to avoid sending files via the remote protocol.
-set GDBFLAGS "${GDBFLAGS} -ex \"set sysroot\""
+set GDBFLAGS "${GDBFLAGS} -iex \"set sysroot\""