Apply fix suggested by @becker33 to copy over relatives includes at env creation

lib/spack/spack/environment/environment.py

@@ -2634,6 +2634,29 @@ def _ensure_env_dir():
 
     shutil.copy(envfile, target_manifest)
 
+    # Copy relative path includes that live inside the environment dir
+    try:
+        manifest = EnvironmentManifestFile(environment_dir)
+    except Exception as e:
+        msg = f"cannot initialize environment, '{environment_dir}' from manifest"
+        raise SpackEnvironmentError(msg) from e
+    else:
+        includes = manifest[TOP_LEVEL_KEY].get("include", [])
+        for include in includes:
+            if os.path.isabs(include):
+                continue
+
+            abspath = pathlib.Path(os.path.normpath(environment_dir / include))
+            if not abspath.is_relative_to(environment_dir):
+                # Warn that we are not copying relative path
+                msg = "Spack will not copy relative include path from outside environment"
+                msg += f" directory: {include}"
+                tty.warn(msg)
+                continue
+
+            orig_abspath = os.path.normpath(envfile.parent / include)
+            shutil.copy(orig_abspath, abspath)
+
 
 class EnvironmentManifestFile(collections.abc.Mapping):
     """Manages the in-memory representation of a manifest file, and its synchronization