fix: exit with the exit code from the first failed task

The integration_test didn't catch this because it doesn't exercise
cmd/run. Perhaps in the future, a better integration test will shell
out.

cmd/run/first.go

@@ -19,9 +19,9 @@ func (f *first[T]) set(val T) {
 	f.mu.Lock()
 	defer f.mu.Unlock()
 
-	f.hasVal = true
 	if !f.hasVal {
 		f.val = val
+		f.hasVal = true
 	}
 }
 

cmd/run/first_test.go

@@ -0,0 +1,17 @@
+package main
+
+import (
+	"testing"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestFirst(t *testing.T) {
+	f := first[string]{}
+	assert.Equal(t, f.get(), "")
+
+	f.set("one")
+	assert.Equal(t, f.get(), "one")
+
+	f.set("two")
+	assert.Equal(t, f.get(), "one")
+}