Use $ANDROID_SERIAL if no selector is specified

Like adb, read the ANDROID_SERIAL environment variable to select a
device by serial if no explicit selection (-s, -d, -e or --tcpip=<addr>)
is provided via the command line.

Fixes #3111 <https://github.com/Genymobile/scrcpy/issues/3111>
PR #3113 <https://github.com/Genymobile/scrcpy/pull/3113>

app/src/cli.c

@@ -660,6 +660,11 @@ static const struct sc_envvar envvars[] = {
         .name = "ADB",
         .text = "Path to adb executable",
     },
+    {
+        .name = "ANDROID_SERIAL",
+        .text = "Device serial to use if no selector (-s, -d, -e or "
+                "--tcpip=<addr>) is specified",
+    },
     {
         .name = "SCRCPY_ICON_PATH",
         .text = "Path to the program icon",