admin: Add CADDY_ADMIN env var to override the default

[francislavoie]

Closes #5317

I took the idea of using init() for this from here:

caddy/cmd/main.go

Lines 40 to 48 in 9623102

	func init() {
	// set a fitting User-Agent for ACME requests
	version, _ := caddy.Version()
	cleanModVersion := strings.TrimPrefix(version, "v")
	ua := "Caddy/" + cleanModVersion
	if uaEnv, ok := os.LookupEnv("USERAGENT"); ok {
	ua = uaEnv + " " + ua
	}
	certmagic.UserAgent = ua

[mholt]

LGTM