shared/install: fix %u expansion when running under sudo

shared/install: fix %u expansion when running under sudo

Test case:
[Install]
DefaultInstance=bond1
WantedBy= foobar-U-%U.device
WantedBy= foobar-u-%u.device

$ sudo systemctl --root=/ enable testing4@.service
(before)
Created symlink /etc/systemd/system/foobar-U-0.device.wants/testing4@bond1.service → /etc/systemd/system/testing4@.service.
Created symlink /etc/systemd/system/foobar-u-zbyszek.device.wants/testing4@bond1.service → /etc/systemd/system/testing4@.service.

(after)
Created symlink /etc/systemd/system/foobar-U-0.device.wants/testing4@bond1.service → /etc/systemd/system/testing4@.service.
Created symlink /etc/systemd/system/foobar-u-root.device.wants/testing4@bond1.service → /etc/systemd/system/testing4@.service.

It doesn't make much sense to use a different user for %U and %u.