mkdir errors without ${HOME}

[attie]

Amaranth / Glasgow now perform hermetic builds, with a totally clean environment (see cb5b0e38).

This causes ${HOME} to expand to nothing, which results in errors making and accessing /.config, /.cache, /.local, which were supposed to be under ${HOME} (perhaps set -u is sensible too?)

For example:

user@99a0a3b7e7d6:~$ env -i /opt/oss-cad-suite/bin/nextpnr-ice40 --version
mkdir: cannot create directory '/.config': Permission denied
mkdir: cannot create directory '/.cache': Permission denied
mkdir: cannot create directory '/.local': Permission denied
/opt/oss-cad-suite/bin/nextpnr-ice40: line 23: /.config/yosyshq/fonts.conf: No such file or directory
"nextpnr-ice40" -- Next Generation Place and Route (Version nextpnr-0.6-29-g54b20457)