fontconfig does not read user specific configuration files,
only ever `access(2)'es them (revealed by strace(1)) but
unlike system-wide configuration never `openat(2)'s them.
$ strace fc-match monospace 2>&1 | grep '\.fonts\.conf'
access("/home/sb/.fonts.conf.d", R_OK) = 0
access("/etc/fonts/~/.fonts.conf.d", R_OK) = -1 ENOENT (No such file or directory)
access("/home/sb/.fonts.conf.d", R_OK) = 0
access("/etc/fonts/~/.fonts.conf.d", R_OK) = -1 ENOENT (No such file or directory)
access("/home/sb/.fonts.conf", R_OK) = -1 ENOENT (No such file or directory)
access("/etc/fonts/~/.fonts.conf", R_OK) = -1 ENOENT (No such file or directory)
access("/home/sb/.fonts.conf", R_OK) = -1 ENOENT (No such file or directory)
access("/etc/fonts/~/.fonts.conf", R_OK) = -1 ENOENT (No such file or directory)