Change auto_now field to auto_now_add

- Build creation date should no longer get erroneously updated

InvenTree/build/migrations/0009_auto_20200210_1032.py

@@ -0,0 +1,18 @@
+# Generated by Django 2.2.9 on 2020-02-10 10:32
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('build', '0008_auto_20200201_1247'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='build',
+            name='creation_date',
+            field=models.DateField(auto_now_add=True),
+        ),
+    ]