Added -u flag to tmux invocations, to force the use of UTF-8 for CLI QR codes

[zenmonkeykstop]

Status

Ready for review

Description of Changes

Fixes #5841

Adds the -u flag to tmux invocations in /etc/profile.d/securedrop_additions.sh - this is required for qr codes to render properly from manage.py.

Testing

on a production (1.8.0-rc1 currently) Focal system:

• log into the application server via ssh app in an Admin Workstation
• as the www-data user, in /var/www/securedrop, run ./manage.py add-admin and confirm that the qr code is rendered as a black square.
• log out from the application server. check out this branch, and copy the file ~/Persistent/securedrop/install_files/securedrop-config-focal/etc/profile.d/securedrop_additions.sh to the application server
• log back into the application server, and overwrite /etc/profile.d/securedrop_additions.sh with the file you just copied, preserving the originals permissions and ownership
• log out of the application server and log back in again with ssh app
• as the www-data user, in /var/www/securedrop, run ./manage.py add-admin and confirm that the qr code is rendered correctly.

Deployment

This is deployed in the securedrop-config package, and should be focal-only.

Checklist

If you made non-trivial code changes:

• I have written a test plan and validated it for this PR

[emkll]

Thanks @zenmonkeykstop those changes resolve the issue for me, good to merge when CI passes.