Given that, lets not rely too heavily on Goblin Builder casting as a "full action" rather than as a "full spell" without some previous precident that the distinction comprises a difference. Are there any other cards worded this way? The solution here may be to make a minor erattum to Goblin Builder to bring it in line with other similar cards.
To me, this is what we absolutely should do.
There are two possibilities:
1) Goblin Builder is meant to use a full action to cast a quick spell as a quick spell.
2) Goblin Builder is meant to use a full action to cast a quick spell as a full spell.
Talking about 1), it is important that the card is worded that way.
If 2) is right, the wording is (slightly) off and might need clarification.
Either way, it is important that Goblin Builder reads full action rather than full spell.