autobuilder: Set HOME, LOGNAME, and USER

Without setting HOME, the shell defaults to the root path (/), which causes autobuilder to fail early during key discovery:

  * Locating (or creating) package signing key pair to use...
mkdir: cannot create directory ‘.abuild’: Permission denied

Closes: #22 (closed) Fixes: 8dd37015 ("Ensure clean environment is always used. Fixes #35 (closed).")