Fix variable name copy/paste error in build-installer.py (GH-13038)

(cherry picked from commit d337169156933eaf732566bf29eb968549ada5e8)

Co-authored-by: cclauss <cclauss@me.com>
diff --git a/Mac/BuildScript/build-installer.py b/Mac/BuildScript/build-installer.py
index 2e3a61e..3ee2fba 100755
--- a/Mac/BuildScript/build-installer.py
+++ b/Mac/BuildScript/build-installer.py
@@ -1538,7 +1538,7 @@
         print(" -- retrying hdiutil create")
         time.sleep(5)
     else:
-        raise RuntimeError("command failed: %s"%(commandline,))
+        raise RuntimeError("command failed: %s"%(cmd,))
 
     if not os.path.exists(os.path.join(WORKDIR, "mnt")):
         os.mkdir(os.path.join(WORKDIR, "mnt"))