Fix "make build" (non-containerized) on macOS
Fix "make build" (non-containerized) on macOS

macOS doesn't ship with the GNU version of `date`, which
causes the command to fail if the `--rfc-3339 ns` format option
is used.

Given that we don't need the build-time with nanosecond precision,
this patch changes the format used, so that the CLI binary can be
built on the host (outside of a container);

Before this change, `make binary` would fail:

    DISABLE_WARN_OUTSIDE_CONTAINER=1 make binary
    WARNING: binary creates a Linux executable. Use cross for macOS or Windows.
    ./scripts/build/binary
    make: *** [binary] Error 1

With this change, the binary can be built on the host:

    DISABLE_WARN_OUTSIDE_CONTAINER=1 make binary
    WARNING: binary creates a Linux executable. Use cross for macOS or Windows.
    ./scripts/build/binary
    Building statically linked build/docker-darwin-amd64

While the previous version formatted (and parsed) the date with nanoseconds precision,
that level of precision is not actually used;

```go
func reformatDate(buildTime string) string {
	t, errTime := time.Parse(time.RFC3339Nano, buildTime)
	if errTime == nil {
		return t.Format(time.ANSIC)
	}
	return buildTime
}
```

Both the old, and new input will yield the same output:

```go
fmt.Println(reformatDate("2019-12-31T13:41:44.846741804+00:00"))
// Tue Dec 31 13:41:44 2019

fmt.Println(reformatDate("2019-12-31T13:41:44Z"))
// Tue Dec 31 13:41:44 2019
```

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

cpuguy83/cli,thaJeztah/cli,docker/cli,cpuguy83/cli,docker/cli,thaJeztah/cli