Add support for nix store remote copying

This is a requirement in order to make Bazel remote execution work. Without it
it's not guaranteed that any necessary nix paths will be available during the
build.

Any local derivation will be copied over to a specified remote nix server which
will also as a Bazel executor. This way any nix paths required during the
build, will be available.

The user will have to provide the `BAZEL_NIX_REMOTE` environment variable. This
should be the name of any entry on the SSH_CONFIG file where all the
authentication details are provided.

e.g

```bash
$ export BAZEL_NIX_REMOTE=nix-server

$ cat ~/.ssh/config
Host nix-server
  Hostname 1.2.3.4
  IdentityFile ~/.ssh/nix-server
  Port 2222
  User nix-user
```

This was done in order to simplify the processes of authentication and keep the
number of configuration options to a minimum.

core/nixpkgs.bzl

@@ -396,22 +396,60 @@ def _nixpkgs_build_file_content(repository_ctx):
     else:
         return None
 
-def _nixpkgs_build_and_symlink(repository_ctx, nix_build, build_file_content):
+def _nixpkgs_build_and_symlink(repository_ctx, nix_build_path, expr_args, build_file_content):
     # Large enough integer that Bazel can still parse. We don't have
     # access to MAX_INT and 0 is not a valid timeout so this is as good
     # as we can do. The value shouldn't be too large to avoid errors on
     # macOS, see https://github.com/tweag/rules_nixpkgs/issues/92.
     timeout = 8640000
     repository_ctx.report_progress("Building Nix derivation")
+
+    nix_host = repository_ctx.os.environ.get('BAZEL_NIX_REMOTE', '')
+    if nix_host:
+        nix_store = "ssh-ng://{host}?max-connections=1".format(host = nix_host)
+        repository_ctx.report_progress("Remote-building Nix derivation")
+        exec_result = execute_or_fail(
+             repository_ctx,
+             [nix_build_path, "--store", nix_store, "--eval-store", "auto"] + expr_args,
+             failure_message = "Cannot build Nix attribute '{}'.".format(
+                 repository_ctx.attr.attribute_path,
+             ),
+             quiet = repository_ctx.attr.quiet,
+             timeout = timeout,
+        )
+        output_path = exec_result.stdout.splitlines()[-1]
+
+        ssh_path = repository_ctx.which("ssh")
+        repository_ctx.report_progress("Creating remote store root")
+        exec_result = execute_or_fail(
+            repository_ctx,
+            [ssh_path] + [nix_host, "nix-store --add-root /nix/var/nix/gcroots/per-user/nix/rules_nixpkgs_{root} -r {path}".format(root = output_path.split('/')[-1], path = output_path) ],
+            failure_message = "Cannot build Nix attribute '{}'.".format(
+                repository_ctx.attr.attribute_path,
+            ),
+            quiet = repository_ctx.attr.quiet,
+            timeout = 10000,
+        )
+
+        nix_path = repository_ctx.which("nix")
+        repository_ctx.report_progress("Downloading Nix derivation")
+        exec_result = repository_ctx.execute(
+            [nix_path, "copy", "--from", nix_store, output_path],
+            quiet = repository_ctx.attr.quiet,
+            timeout = 10000,
+        )
+
     exec_result = execute_or_fail(
         repository_ctx,
-        nix_build,
+        [nix_build_path] + expr_args,
         failure_message = "Cannot build Nix derivation for package '@{}'.".format(repository_ctx.name),
         quiet = repository_ctx.attr.quiet,
         timeout = timeout,
     )
     output_path = exec_result.stdout.splitlines()[-1]
 
+    repository_ctx.report_progress("Creating local folders")
+
     # ensure that the output is a directory
     test_path = repository_ctx.which("test")
     execute_or_fail(
@@ -542,9 +580,8 @@ def _nixpkgs_package_impl(repository_ctx):
         "nix-build",
         extra_msg = "See: https://nixos.org/nix/",
     )
-    nix_build = [nix_build_path] + expr_args
 
-    _nixpkgs_build_and_symlink(repository_ctx, nix_build, build_file_content)
+    _nixpkgs_build_and_symlink(repository_ctx, nix_build_path, expr_args, build_file_content)
 
 _nixpkgs_package = repository_rule(
     implementation = _nixpkgs_package_impl,