install: remove dependence on `pi` user

[ES-Alexander]

Raised in #1072, and relevant because:

• not every OS has pi as the default user (e.g. Ubuntu)
• support for non-Raspberry Pi boards will likely require support for alternative user names

Should either:

• change script to use $USER or $USERNAME instead of pi, or
• set a target_user variable (or similar) at the top which defaults to pi (or $USER) and allows specifying something else with a command-line argument

[Jollerprutt]

The Raspberry Pi foundation has announced they are dropping pi as the default user, and that there will no longer be any default username. So this issue will soon become more urgent as people start using the newer images.

[ES-Alexander]

Will also need to change commonwealth

[ES-Alexander]

support for non-Raspberry Pi boards...

Case in point.