Fixes <https://bugs.gnu.org/46362>.
Reported by Zhu Zihao <all_but_last@163.com>.
Previously, backtraces due to uncaught exceptions would always to go
file descriptor 2; the daemon would read it and error out with something
like:
error: got unexpected path `Backtrace:' from substituter
This patch fixes that by ensuring backtraces are properly displayed on
file descriptor 4.
* guix/scripts/substitute.scm (with-redirected-error-port): New macro.
(guix-substitute): Use 'with-redirected-error-port' instead of 'parameterize'.