Add sudo to the lines in install-sessions.sh that need it so you don't need to sudo the whole thing

Summary:
Because this script copies things to /, it needs to be run with sudo. But two of the
copy operations also copy things to your home directory, and if you sudo the whole
script, this results in those files being copied with root ownership, which can cause
problems later.

This patch prepends sudo to just the copy operations that need it so that the ones
that don't aren't run with inappropriately elevated permissions.

Test Plan:

• Run ~/kde/build/plasma-workspace/login-sessions/install-sessions.sh
• Get prompted for the root password
• Enter it
• See that plasma-dev-prefix.sh and startplasma-dev.sh in ~/kde/usr/lib64/libexec/ have user ownership, not root ownership

Reviewers: apol, Plasma

Reviewed By: apol

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24677