go.mod: retract v0.5.0

Since the SDK in v0.5.0 contains a release statement in its go.mod,
this makes it impossible to go install packer-sdc from a plugin.

Since those plugins rely on their Makefile to do this, we cannot change
them all at once, so we fixed the problem in the SDK code itself, and
v0.5.0 is broken, so we retract it in this commit.

go.mod

@@ -119,3 +119,5 @@ require (
 )
 
 go 1.19
+
+retract v0.5.0 // v0.5.0 of the SDK was broken because of the replace statement for go-cty