restore-backup: Open backup tarball as root.
restore-backup: Open backup tarball as root.

Fixes permission errors when running restore-backup on a tarball
inaccessible to the zulip user.

Fixes #12125.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulipchat.com>

rishig/zulip,timabbott/zulip,rht/zulip,andersk/zulip,rht/zulip,hackerkid/zulip,eeshangarg/zulip,showell/zulip,hackerkid/zulip,shubhamdhama/zulip,synicalsyntax/zulip,brainwane/zulip,tommyip/zulip,showell/zulip,andersk/zulip,hackerkid/zulip,tommyip/zulip,rishig/zulip,timabbott/zulip,andersk/zulip,showell/zulip,synicalsyntax/zulip,punchagan/zulip,timabbott/zulip,andersk/zulip,synicalsyntax/zulip,showell/zulip,kou/zulip,timabbott/zulip,shubhamdhama/zulip,showell/zulip,shubhamdhama/zulip,punchagan/zulip,showell/zulip,kou/zulip,rishig/zulip,andersk/zulip,zulip/zulip,kou/zulip,synicalsyntax/zulip,hackerkid/zulip,brainwane/zulip,shubhamdhama/zulip,hackerkid/zulip,tommyip/zulip,rishig/zulip,andersk/zulip,zulip/zulip,brainwane/zulip,brainwane/zulip,zulip/zulip,kou/zulip,timabbott/zulip,rht/zulip,eeshangarg/zulip,shubhamdhama/zulip,punchagan/zulip,rishig/zulip,kou/zulip,tommyip/zulip,tommyip/zulip,punchagan/zulip,showell/zulip,eeshangarg/zulip,hackerkid/zulip,punchagan/zulip,synicalsyntax/zulip,rht/zulip,brainwane/zulip,brainwane/zulip,rht/zulip,rishig/zulip,punchagan/zulip,eeshangarg/zulip,timabbott/zulip,zulip/zulip,synicalsyntax/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,tommyip/zulip,zulip/zulip,brainwane/zulip,zulip/zulip,shubhamdhama/zulip,synicalsyntax/zulip,kou/zulip,rht/zulip,eeshangarg/zulip,shubhamdhama/zulip,punchagan/zulip,timabbott/zulip,rht/zulip,hackerkid/zulip,tommyip/zulip,rishig/zulip,eeshangarg/zulip,kou/zulip