gdb-add-index: disable debuginfod

gdb-add-index may trigger debuginfod's first-use notice.  The notice
is misleading in this case.  It instructs the user to modify .gdbinit
in order to permanently enable/disable debuginfod but gdb-add-index
invokes gdb with -nx which ignores .gdbinit.

Additionally debuginfod is not needed for gdb-add-index since the
symbol file is given as an argument and should already be present
locally.

Fix this by disabling debuginfod when gdb-add-index invokes gdb.