Do not attempt to execute the output of update-initramfs

Backticks substitute their output, and if executes its first
argument, which leads to both losing the output of update-initramfs
and an error message.

In fact, the whole debug output isn't needed.

Signed-off-by: Andrew Shadura <andrew.shadura@collabora.co.uk>
Reviewed-by: Denis Pynkin <denis.pynkin@collabora.com>
Differential Revision: https://phabricator.apertis.org/D7449