[teleport-update] Add link subcommand

[sclevine]

This PR adds the link subcommand to the teleport-update command.

This subcommand will allow the teleport RPM and DEB packages to co-exist with versions of Teleport that are installed by the teleport-update binary, by ensuring that Teleport binaries installed by the package into /usr/local/teleport-system are always linked to /usr/local when no teleport-update-managed installation is present.

The link subcommand links the system (package) installation of Teleport from /usr/local/teleport-system to /usr/local/{bin,lib/systemd/system}. This subcommand will be executed by the teleport package's postinst script to ensure that when auto-updates are disabled, the package continues to function as it does today. See the RFD linked below for more details.

Note that the Teleport package will have its files moved into /usr/local/teleport-system in a future PR. This command is not useful until this packaging change is complete.

This PR also fixes the following small bugs related to linking:

• teleport.service must be copied instead of linked, since systemctl enable resolves multi-level links
• enterprise installations are broken due to the tarball containing teleport-ent as the top-level directory

This is the sixth in a series of PRs implementing teleport-update:
Update Command: #48244
Reloading with rollbacks: #47929
Linking: #47879
Enable Command: #47565
Initial scaffolding PR: #46418

The teleport-update binary will be used to enable, disable, and trigger automatic Teleport agent updates. The new auto-updates system manages a local installation of the cluster-specified version of Teleport stored in /var/lib/teleport/versions.

RFD: #47126
Goal (internal): https://github.com/gravitational/cloud/issues/10289

Examples:

bash$ sudo ./teleport-update update
2024-11-12T23:02:56Z INFO [UPDATER]   Update available. Initiating update. target_version:17.0.0-beta.1 active_version:18.0.0-dev.updates.2 agent/updater.go:435
2024-11-12T23:03:56Z INFO [UPDATER]   Version already present. version:17.0.0-beta.1 agent/installer.go:140
2024-11-12T23:03:56Z INFO [UPDATER]   Target version successfully installed. target_version:17.0.0-beta.1 agent/updater.go:510
2024-11-12T23:03:56Z INFO [UPDATER]   Teleport gracefully reloaded. agent/process.go:74
2024-11-12T23:03:56Z INFO [UPDATER]   Backup version set. backup_version:18.0.0-dev.updates.2 agent/updater.go:535
2024-11-12T23:03:56Z INFO [UPDATER]   Configuration updated. agent/updater.go:450

bash$ sudo ./teleport-update link
2024-11-12T23:11:02Z INFO [UPDATER]   Automatic updates enabled. Skipping system package link. active_version:17.0.0-beta.1 agent/updater.go:626

bash$ sudo ./teleport-update disable
bash$ sudo ./teleport-update link
ERROR: failed to link system package installation: failed to find Teleport binary directory: open /usr/local/teleport-system/bin: no such file or directory

bash$ sudo cp -r /var/lib/teleport/versions/17.0.0-beta.1 /usr/local/teleport-system
bash$ sudo ./teleport-update link
2024-11-13T01:42:18Z WARN [UPDATER]   Automatic updates disabled, but a non-package version of Teleport is linked. active_version:17.0.0-beta.1 agent/updater.go:633

bash$ sudo rm /usr/local/bin/{teleport,tsh,tctl,tbot,fdpass-teleport}
bash$ sudo rm /usr/local/lib/systemd/system/teleport.service
bash$ sudo ./teleport-update link
2024-11-13T01:47:01Z INFO [UPDATER]   Successfully linked system package installation. agent/updater.go:638

bash$ ls -l /usr/local/bin/
lrwxrwxrwx  1 root root   46 Nov 13 01:47 fdpass-teleport -> /usr/local/teleport-system/bin/fdpass-teleport
lrwxrwxrwx  1 root root   35 Nov 13 01:47 tbot -> /usr/local/teleport-system/bin/tbot
lrwxrwxrwx  1 root root   35 Nov 13 01:47 tctl -> /usr/local/teleport-system/bin/tctl
lrwxrwxrwx  1 root root   39 Nov 13 01:47 teleport -> /usr/local/teleport-system/bin/teleport
lrwxrwxrwx  1 root root   34 Nov 13 01:47 tsh -> /usr/local/teleport-system/bin/tsh
bash$ ls -l /usr/local/lib/systemd/system
-rw-r--r-- 1 root root  435 Nov 13 01:47 teleport.service

[sclevine]

Could use one more review when someone has a chance 🙂