roachprod: fix target path when getting cluster artifacts.

[renatolabs]

A bug was introduced in the computation of the local path where
cluster artifacts are downloaded: instead of prefixing the path with
the node ID, the list of nodes would be used instead. This results
in (other than weird looking paths, such as [1 2 3 4].logs) only the
logs for the last node being downloaded.

This commit fixes the issue by using the node ID correctly.

Release note: None.

[cockroach-teamcity]

This change is 

[renatolabs]

Using Tobi's reproduction steps on this branch:

$ ./bin/roachprod create -n 3 local
18:30:45 roachprod.go:1266: Creating cluster local with 3 nodes
$ ./bin/roachprod ssh local -- mkdir -p logs
local: mkdir -p logs 3/3
   1: 
   2: 
   3: 
$ ./bin/roachprod get local logs
18:31:09 cluster_synced.go:1839: local: getting logs logs on nodes [1 2 3]
   1: done
   2: done
   3: done
$ ls | grep logs
1.logs
2.logs
3.logs

[renatolabs]

bors r=srosenberg

[craig]

Build failed (retrying...):

• GitHub CI (Cockroach)

[craig]

Build succeeded:

• GitHub CI (Cockroach)