secure_less.sh: Fix factory install log viewing

Starting from ChromeOS 7051.0.0, using 'su' to invoke "busybox less"
will dump the file to the output and exit immediately (like cat).
This makes infinitely dumping factory_install.log to TTY3.

Replace 'su' with 'sudo' can fix this issue.

BUG=None
TEST=Boot factory install shim and switch to TTY3, can see factory
     install log.

Change-Id: I8a4486252993cae8ff9de6c4d0c30f8d377adf76
Reviewed-on: https://chromium-review.googlesource.com/273450
Commit-Queue: Bowgo Tsai <bowgotsai@chromium.org>
Tested-by: Bowgo Tsai <bowgotsai@chromium.org>
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
1 file changed