Prospective fix for git lfs installation

Keys used to sign package repositories tend to have expiry dates and
therefore need to be changed from time to time. Therefore hardcoding
the key we expect here will break (as it does right now) and it's better
to retrieve the expected key via a secure transport.

The old key - per
http://keyserver.ubuntu.com/pks/lookup?search=0xC2E73424D59097AB&op=vindex
- will expire on the 12 of January. That's probably why they changed the
key used to sign the repo before the expiry.

Change-Id: I39d082a6e12731b7eb0a8214a500101ca24f8af4
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@qt.io>
Reviewed-by: Heikki Halmet <heikki.halmet@qt.io>
(cherry picked from commit 0b18f12f99)
Reviewed-by: Jani Heikkinen <jani.heikkinen@qt.io>

coin/provisioning/qtci-linux-Ubuntu-16.04-x86_64/02-git_lfs.sh

@@ -37,7 +37,7 @@
 
 set -ex
 
+curl -L https://packagecloud.io/github/git-lfs/gpgkey | sudo apt-key add -
 sudo apt-add-repository 'deb https://packagecloud.io/github/git-lfs/ubuntu/ xenial main'
-sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 37BBEE3F7AD95B3F
 sudo apt update
 sudo apt install git-lfs