packer: remove ineffasign pointed assigns/decls

Some of the variables we create are flagged by our linters as
ineffective assignments, which makes sense as those are generally fed by
code below, so we don't need to use the declaration/assignation syntax
(:=) but instead can fall back to using var with a type to get the zero
value of the declared entity.

datasource/hcp-packer-image/data.go

@@ -188,7 +188,7 @@ func (d *Datasource) Execute() (cty.Value, error) {
 			iteration.ID)
 	}
 
-	output := DatasourceOutput{}
+	var output DatasourceOutput
 
 	cloudAndRegions := map[string][]string{}
 	for _, build := range iteration.Builds {

packer/core.go

@@ -939,7 +939,6 @@ func (c *Core) renderVarsRecursively() (*interpolate.Context, error) {
 				}
 			}
 		}
-		deleteKeys = []string{}
 	}
 
 	if !changed && shouldRetry {

post-processor/manifest/post-processor.go

@@ -141,7 +141,7 @@ func (p *PostProcessor) PostProcess(ctx context.Context, ui packersdk.Ui, source
 	defer os.Remove(lockFilename)
 
 	// Read the current manifest file from disk
-	contents := []byte{}
+	var contents []byte
 	if contents, err = os.ReadFile(p.config.OutputPath); err != nil && !os.IsNotExist(err) {
 		return source, true, true, fmt.Errorf("Unable to open %s for reading: %s", p.config.OutputPath, err)
 	}