fix(#17): src should sync with tag

[williamfzc]

to fix #17

[letompouce]

The $SCRCPY_VER tag is used in order to download a binary that will be used as some kind of a cache.
You can notice that this binary is passed to the meson build as prebuilt_server=/scrcpy-server.

The git clone step, however, will download the latest code - i.e. from HEAD - and will compile that.
By passing $SCRCPY_VER to git clone, you end up cloning and building the exact same version than the binary that was just downloaded :-|