switch-to-configuration: Handle failure to read /proc/1/exe

It's not entirely clear why this happens, but sometimes /proc/1/exe
returns a bogus value, like
/ar3a3j6b9livhy5fcfv69izslhgk4gcz-systemd-217/lib/systemd/systemd. In
any case, we can just conservatively assume that we need to restart
systemd when this happens.

Fixes #10261.

nixos/modules/system/activation/switch-to-configuration.pl

···
 
 
 # Should we have systemd re-exec itself?
-my $prevSystemd = abs_path("/proc/1/exe") or die;
+my $prevSystemd = abs_path("/proc/1/exe") // "/unknown";
 my $newSystemd = abs_path("@systemd@/lib/systemd/systemd") or die;
 my $restartSystemd = $prevSystemd ne $newSystemd;