Update BCR presubmit.yml script

Copied from bazelbuild/bazel-central-registry#1022

.bcr/core/presubmit.yml

@@ -6,10 +6,11 @@ bcr_test_module:
     run_tests:
       name: "Run test module"
       platform: ${{ platform }}
+      environment:
+        # The Nix installer updates ~/.profile to set PATH. However, the Bazel
+        # CI setup seems to disregard ~/.profile. So we set PATH manually here.
+        PATH: /var/lib/buildkite-agent/.nix-profile/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
       shell_commands:
-        - apt update
-        - apt install --yes nix-setup-systemd
-        - nix-channel --add https://nixos.org/channels/nixpkgs-unstable nixpkgs
-        - nix-channel --update
+        - curl -L https://nixos.org/nix/install | sh -s -- --no-daemon
       test_targets:
         - "//..."