update build/test instructions in dev docs

docs/dev/README.rst

@@ -16,13 +16,13 @@ The test system uses `Nix`_ which must be installed before any tests can be run.
 
 Unit tests are run using this command::
 
-  $ nix-build nixos/unit-tests.nix
+  $ nix-build --attr unit-tests
 
 Unit tests are also run on CI.
 
 The system tests are run using this command::
 
-  $ nix-build nixos/system-tests.nix
+  $ nix-build --attr system-tests
 
 The system tests boot QEMU VMs which prevents them from running on CI at this time.
 The build requires > 10 GB of disk space,
@@ -32,7 +32,7 @@ try `raising the number of retries <https://whetstone.privatestorage.io/privates
 
 It is also possible go through the testing script interactively - useful for debugging::
 
-  $ nix-build -A private-storage.driver nixos/system-tests.nix
+  $ nix-build --attr system-tests.private-storage.driver
 
 This will give you a result symlink in the current directory.
 Inside that is bin/nixos-test-driver which gives you a kind of REPL for interacting with the VMs.