client/web: fill devMode from an env var

Avoids the need to pipe a web client dev flag through the tailscaled
command.

Updates tailscale/corp#14335

Signed-off-by: Sonia Appasamy <sonia@tailscale.com>

cmd/tailscale/cli/web.go

@@ -41,7 +41,6 @@ Tailscale, as opposed to a CLI or a native app.
 		webf := newFlagSet("web")
 		webf.StringVar(&webArgs.listen, "listen", "localhost:8088", "listen address; use port 0 for automatic")
 		webf.BoolVar(&webArgs.cgi, "cgi", false, "run as CGI script")
-		webf.BoolVar(&webArgs.dev, "dev", false, "run web client in developer mode [this flag is in development, use is unsupported]")
 		webf.StringVar(&webArgs.prefix, "prefix", "", "URL prefix added to requests (for cgi or reverse proxies)")
 		return webf
 	})(),
@@ -51,7 +50,6 @@ Tailscale, as opposed to a CLI or a native app.
 var webArgs struct {
 	listen string
 	cgi    bool
-	dev    bool
 	prefix string
 }
 
@@ -104,7 +102,6 @@ func runWeb(ctx context.Context, args []string) error {
 
 	webServer, err := web.NewServer(web.ServerOpts{
 		Mode:        cliServerMode,
-		DevMode:     webArgs.dev,
 		CGIMode:     webArgs.cgi,
 		PathPrefix:  webArgs.prefix,
 		LocalClient: &localClient,