Fix missing registration for environments rules in pants.core. (#20444)

Fixes (hopefully) pantsbuild/pantsbuild.org#133

Not sure how the `docs/reference/help-all.json` is updated in the docs
repo, but locally updating it and generating the references docs I got
this diff with this change:
```
❯ git diff docs/reference/subsystems/environments-preview.mdx
diff --git a/docs/reference/subsystems/environments-preview.mdx b/docs/reference/subsystems/environments-preview.mdx
index 8231798..c538672 100644
--- a/docs/reference/subsystems/environments-preview.mdx
+++ b/docs/reference/subsystems/environments-preview.mdx
@@ -11,7 +11,7 @@ import styles from "@site/src/components/reference/styles.module.css";

 A highly experimental subsystem to allow setting environment variables and executable search paths for different environments, e.g. macOS vs. Linux.

-Backend: <span className="color--primary">``</span>
+Backend: <span className="color--primary">`pants.core`</span>

 Config section: <span className="color--primary">`[environments-preview]`</span>
```

Technically, this is a bug fix, but in practice it only affects
docs/help data.

Author: kaos

src/python/pants/core/register.py

@@ -44,6 +44,7 @@
     adhoc_binaries,
     archive,
     config_files,
+    environments,
     external_tool,
     source_files,
     stripped_source_files,
@@ -90,6 +91,7 @@ def rules():
         *anonymous_telemetry.rules(),
         *archive.rules(),
         *config_files.rules(),
+        *environments.rules(),
         *external_tool.rules(),
         *git.rules(),
         *source_files.rules(),