fix(plugin): add HELM_HOME back

This allows Helm 2 plugins that used HELM_HOME as a scratchpad to continue to work the same in Helm 3.

Signed-off-by: Matthew Fisher <matt.fisher@microsoft.com>

pkg/plugin/plugin.go

@@ -230,6 +230,7 @@ func SetupPluginEnv(settings helm_env.EnvSettings,
 		"HELM_PATH_CACHE":            helmpath.CachePath(),
 		"HELM_PATH_CONFIG":           helmpath.ConfigPath(),
 		"HELM_PATH_DATA":             helmpath.DataPath(),
+		"HELM_HOME":                  helmpath.DataPath(), // for backwards compatibility with Helm 2 plugins
 	} {
 		os.Setenv(key, val)
 	}