Sync after installing the boot loader

Previously we synced just before calling switch-to-configuration.
That prevents corruption of the Nix store, but it can leave the boot
loader configuration and kernel files in /boot corrupted.  So do the
sync after installing the boot loader.

modules/installer/tools/nixos-rebuild.sh

@@ -191,9 +191,6 @@ fi
 # If we're not just building, then make the new configuration the boot
 # default and/or activate it now.
 if [ "$action" = switch -o "$action" = boot -o "$action" = test ]; then
-    # Just in case the new configuration hangs the system, do a sync now.
-    sync
-
     $pathToConfig/bin/switch-to-configuration "$action"
 fi
 

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

@@ -33,9 +33,13 @@ openlog("nixos", "", LOG_USER);
 # Install or update the bootloader.
 if ($action eq "switch" || $action eq "boot") {
     system("@installBootLoader@ $out") == 0 or exit 1;
-    exit 0 if $action eq "boot";
 }
 
+# Just in case the new configuration hangs the system, do a sync now.
+system("sync") unless ($ENV{"NIXOS_NO_SYNC"} // "") eq "1";
+
+exit 0 if $action eq "boot";
+
 # Check if we can activate the new configuration.
 my $oldVersion = read_file("/run/current-system/init-interface-version", err_mode => 'quiet') // "";
 my $newVersion = read_file("$out/init-interface-version");