nit: add missing diskSelector to node_filesystem_files_free

IIUC, this doesn't change the behavior of this query at all (Prometheus
will still only pick values for `node_filesystem_files_free` and
`node_filesystem_files` where all the labels match), but perhaps
makes it cleaner to read.

I'm extremely new to PromQL, and I'm mostly filing this as a learning
opportunity. Are there best practices around this sort of thing? Do we
like the way this was written, and should I change our other expressions
ot match (some of the other expressions use `${diskSelector}` twice).

build/pluto/prometheus/exporters/node.nix

@@ -68,7 +68,7 @@
                   {
                     alert = "PartitionLowInodes";
                     expr = ''
-                      node_filesystem_files_free / node_filesystem_files{${diskSelector}} * 100 < 10
+                      node_filesystem_files_free{${diskSelector}} / node_filesystem_files{${diskSelector}} * 100 < 10
                     '';
                     for = "30m";
                     labels.severity = "warning";