internal/build: include git-date on detached head

[holiman]

When we are building in detached head, we cannot easily obtain the same information as we can if we're in non-detached head.

However, one thing we can obtain is the git-hash and git-date. Currently, we omit to include the git-date into the build-info, which causes problem for reproducable builds which are on a detached head. This PR should fix it.

---

Can be tested locally by adding an early return in ci.go and then do go run build/ci.go install , check out a specific git commit and add some printouts to see the env.

func doInstall(cmdline []string) {
	var (
		dlgo       = flag.Bool("dlgo", false, "Download Go and build with it")
		arch       = flag.String("arch", "", "Architecture to cross build for")
		cc         = flag.String("cc", "", "C compiler to cross build with")
		staticlink = flag.Bool("static", false, "Create statically-linked executable")
	)
	flag.CommandLine.Parse(cmdline)
	env := build.Env()
        return

HT to @vivi365 for finding

[MariusVanDerWijden]

LGTM