Test-Grid: Use symlinks to config instead of copying bits of the config on every service start?

[hacklschorsch]

Can we now use the symlink that the original NixOS module says we cannot?

Introducers:

infrastructure/testgrid.tahoe-lafs.org/tahoe-service.nix

Lines 252 to 256 in 3046318

	# XXX I thought that a symlink would work here, but it doesn't, so
	# we must do this on every prestart. Fixes welcome.
	# rm $STATE_DIRECTORY/tahoe.cfg
	# ln -s /etc/tahoe-lafs/introducer-${node}.cfg $STATE_DIRECTORY/tahoe.cfg
	cp /etc/tahoe-lafs/introducer-"${node}".cfg $STATE_DIRECTORY/tahoe.cfg

Storage nodes:

infrastructure/testgrid.tahoe-lafs.org/tahoe-service.nix

Lines 358 to 362 in 3046318

	# XXX I thought that a symlink would work here, but it doesn't, so
	# we must do this on every prestart. Fixes welcome.
	# rm $STATE_DIRECTORY/tahoe.cfg
	# ln -s /etc/tahoe-lafs/${lib.escapeShellArg node}.cfg $STATE_DIRECTORY/tahoe.cfg
	cp /etc/tahoe-lafs/${lib.escapeShellArg node}.cfg $STATE_DIRECTORY/tahoe.cfg