Fix syntax warning with terraform 0.13

[fpletz]

Terraform 0.13 will throw the following warning:

Warning: Interpolation-only expressions are deprecated
on .terraform/modules/dbpool.deploy_nixos/deploy_nixos/main.tf line
104, in data "external" "nixos-instantiate":
104: "--argstr", "system", "${var.target_system}"
Terraform 0.11 and earlier required all non-constant expressions to be
provided via interpolation syntax, but this pattern is now deprecated. To
silence this warning, remove the "${ sequence from the start and the }"
sequence from the end of this expression, leaving just the inner expression.
Template interpolation syntax is still used to construct strings from
expressions when the template includes multiple interpolation sequences or a
mixture of literal strings and interpolations. This deprecation applies only
to templates that consist entirely of a single interpolation sequence.
(and 3 more similar warnings elsewhere)

Since this syntax is already supported in 0.12 and everybody should have
upgraded to that till now, this change should be safe.

[zimbatm]

thanks Franz! :)