Pass along the configuration directory for cache plugins

The cache plugin directory is controlled by environment variable.  If
the environment variable is set in the `pelican` environment, pass its
value along to xrootd.

Simplifies testing in environments where the plugin is not installed
as root.

xrootd/launch.go

@@ -23,6 +23,7 @@ package xrootd
 import (
 	"context"
 	_ "embed"
+	"os"
 	"path/filepath"
 	"regexp"
 	"strconv"
@@ -87,6 +88,9 @@ func makeUnprivilegedXrootdLauncher(daemonName string, configPath string, isCach
 			"XRD_PELICANCLIENTCERTFILE=" + filepath.Join(xrootdRun, "copied-tls-creds.crt"),
 			"XRD_PELICANCLIENTKEYFILE=" + filepath.Join(xrootdRun, "copied-tls-creds.crt"),
 		}
+		if confDir := os.Getenv("XRD_PLUGINCONFDIR"); confDir != "" {
+			result.ExtraEnv = append(result.ExtraEnv, "XRD_PLUGINCONFDIR="+confDir)
+		}
 	}
 	return
 }