Fix TERM fallback in generated .zshrc

[seven-mile]

#30 made the TERM default conditional, but the line lives inside zshrc_template()'s
unquoted heredoc, so $TERM is expanded while ~/.zshrc is generated instead of being
written literally. The install-time value gets baked in:

[ -z "" ] && export TERM=xterm          # TERM unset at install: always overrides TERM
[ -z "xterm" ] && export TERM=xterm     # TERM set at install: fallback never fires

During docker build TERM is usually unset, so the guard is always true and the line is
equivalent to the unconditional export TERM=xterm that #30 replaced — passing
--env=TERM=xterm-256color still has no effect.

Escaping the $ defers the check to shell startup, so ~/.zshrc gets
[ -z "$TERM" ] && export TERM=xterm regardless of the build environment.