Fix manager installation from repository issues

[mikliapko]

While running CI pipelines without manager-server, manager-agent URLs specification, the job takes the repo, puts it into /etc/apt/sources.list.d/scylla.manager.list and tries to install the packages from repo. It fails with the following error:

scyllaadm@manager-regression-manager--db-node-45179409-1:~$ sudo apt update
Hit:1 http://us-east-1.ec2.archive.ubuntu.com/ubuntu focal InRelease
Hit:2 http://us-east-1.ec2.archive.ubuntu.com/ubuntu focal-updates InRelease
Hit:3 http://us-east-1.ec2.archive.ubuntu.com/ubuntu focal-backports InRelease
Hit:4 http://security.ubuntu.com/ubuntu focal-security InRelease
Hit:5 https://downloads.scylladb.com/unstable/scylla-enterprise/enterprise-2022.2/deb/unified/2024-03-28T09:48:47Z/scylladb-2022.2 stable InRelease
Get:6 https://downloads.scylladb.com/manager/deb/unstable/unified-deb/master/2024-05-09T10:27:20Z stable InRelease [7549 B]
Err:6 https://downloads.scylladb.com/manager/deb/unstable/unified-deb/master/2024-05-09T10:27:20Z stable InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 87722433EBB454AE
Reading package lists... Done
W: GPG error: https://downloads.scylladb.com/manager/deb/unstable/unified-deb/master/2024-05-09T10:27:20Z stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 87722433EBB454AE
E: The repository 'https://downloads.scylladb.com/manager/deb/unstable/unified-deb/master/2024-05-09T10:27:20Z stable InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

Jenkins job: https://jenkins.scylladb.com/view/staging/job/scylla-staging/job/mikita/job/sct/job/manager-3.2/job/ubuntu22-sanity-test/4/

[mikliapko]

Closed in the scylladb/scylla-cluster-tests#7429