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>

README.md

@@ -448,6 +448,9 @@ scrcpy --serial 0123456789abcdef
 scrcpy -s 0123456789abcdef  # short version
 ```
 
+The serial may also be provided via the environment variable `ANDROID_SERIAL`
+(also used by `adb`).
+
 If the device is connected over TCP/IP:
 
 ```bash