fix: cli variables should take priority over aaskfile defaults (#2589)

When using `task FOO=bar` with a Taskfile containing
`FOO: '{{.FOO | default "foo"}}'`, the CLI value was being
overwritten by the Taskfile default.

Split the variable merging into two steps:
1. Merge CLI variables (FOO=bar) first so they override Taskfile vars
2. ReverseMerge special variables (CLI_ARGS, CLI_FORCE, etc.) so
   they're available for templating in Taskfile vars

Fixes regression introduced in #2403.

Co-authored-by: Timothy Rule <timothy.rule@gmail.com>

executor_test.go

@@ -263,6 +263,23 @@ func TestVars(t *testing.T) {
 			task.WithSilent(true),
 		),
 	)
+	NewExecutorTest(t,
+		WithName("cli-var-priority-default"),
+		WithExecutorOptions(
+			task.WithDir("testdata/vars"),
+			task.WithSilent(true),
+		),
+		WithTask("cli-var-priority"),
+	)
+	NewExecutorTest(t,
+		WithName("cli-var-priority-override"),
+		WithExecutorOptions(
+			task.WithDir("testdata/vars"),
+			task.WithSilent(true),
+		),
+		WithTask("cli-var-priority"),
+		WithVar("CLI_VAR", "from_cli"),
+	)
 }
 
 func TestRequires(t *testing.T) {