bake: move SOURCE_DATE_EPOCH parsing to overrides

Previously, when directly modifying the args map when reading targets,
we could end up in a scenario where bake tests that compare arg maps
would fail if SOURCE_DATE_EPOCH was set in the environment.

This patch prevents this failure by setting the SOURCE_DATE_EPOCH at the
command level (which isn't injected into tests as well), ensuring that
we test correctly even when SOURCE_DATE_EPOCH is set in the environment.

Signed-off-by: Justin Chadwell <me@jedevc.com>

bake/bake.go

@@ -140,19 +140,6 @@ func ReadTargets(ctx context.Context, files []File, targets, overrides []string,
 		}
 	}
 
-	// Propagate SOURCE_DATE_EPOCH from the client env.
-	// The logic is purposely duplicated from `build/build`.go for keeping this visible in `bake --print`.
-	if v := os.Getenv("SOURCE_DATE_EPOCH"); v != "" {
-		for _, f := range m {
-			if f.Args == nil {
-				f.Args = make(map[string]*string)
-			}
-			if _, ok := f.Args["SOURCE_DATE_EPOCH"]; !ok {
-				f.Args["SOURCE_DATE_EPOCH"] = &v
-			}
-		}
-	}
-
 	return m, n, nil
 }