nushell: Don't assume that XDG_RUNTIME_DIR is set

tree/.config/nushell/env.nu

@@ -86,11 +86,13 @@ $env.PATH = ($env.PATH | split row (char esep) | prepend [
 ])
 
 # auto-detect which ssh-agent to use
+if ("XDG_RUNTIME_DIR" in $env) {
 	if (which tkey-ssh-agent | is-empty) {
 		$env.SSH_AUTH_SOCK = $"($env.XDG_RUNTIME_DIR)/ssh-agent.socket"
 	} else {
 		$env.SSH_AUTH_SOCK = $"($env.XDG_RUNTIME_DIR)/tkey-ssh-agent/sock"
 	}
+}
 
 $env.GPG_TTY = (tty)
 $env.BROWSER = "firefox"