install: remove dependence on `pi` user

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