ci: use nix copy instead of nix-copy-closure

Also use `ssh-ng://` protocol implementation instead of `ssh://`.
Might fix the following error:
```
error (ignored): error: reached end of FramedSource
error: sized: unexpected end-of-file
error: unexpected end-of-file
```
For more details see: https://github.com/NixOS/nix/issues/6495

Signed-off-by: Jakub Sokołowski <jakub@status.im>

ci/Jenkinsfile.nix-cache

@@ -98,8 +98,8 @@ pipeline {
           nix.shell("""
               find /nix/store/ -mindepth 1 -maxdepth 1 -type d \
                 -not -name '*.links' -and -not -name '*-status-react-*' \
-                | xargs nix-copy-closure -v \
-                  --to ${params.NIX_CACHE_USER}@${params.NIX_CACHE_HOST}
+                | xargs nix copy \
+                  --to ssh-ng://${params.NIX_CACHE_USER}@${params.NIX_CACHE_HOST}
             """,
             pure: false
           )

nix/nix.conf

@@ -11,3 +11,5 @@ keep-derivations = true
 keep-outputs = true
 # Extra isolation for network and filesystem, doesn't work on MacOS
 build-use-sandbox = false
+# Enable Nix v2 interface.
+experimental-features = nix-command