tests: only provide template values when used

As otherwise the `.golden` values can not be automatically updated using
`-update` as documented in `CONTRIBUTING.md`.

Also ensure we do not use `defer` but rather `t.Cleanup` in tests, as
this will always be called even if e.g. `t.Fatal` absruptly stops the
test.

Signed-off-by: Hidde Beydals <hello@hidde.co>

cmd/flux/build_artifact_test.go

@@ -54,7 +54,7 @@ data:
 			tmpFile, err := saveReaderToFile(strings.NewReader(tt.string))
 			g.Expect(err).To(BeNil())
 
-			defer os.Remove(tmpFile)
+			t.Cleanup(func() { _ = os.Remove(tmpFile) })
 
 			b, err := os.ReadFile(tmpFile)
 			if tt.expectErr {