Use POSIX (I think) USER rather than calling whoami

dotfiles/bash/head.bashrc

   "${HOME}/.bash/colors.bashrc" \
   "${HOME}/.bash/defaults.bashrc" \
   "${HOME}/.bash/main.bashrc" \
-  "${HOME}/.bash/user/$(whoami).bashrc" \
+  "${HOME}/.bash/user/$USER.bashrc" \
   "${HOME}/.bash/host/$(hostname -s).bashrc" \
   "${HOME}/.bash/post.bashrc";
 do