Support using wget instead of curl in fetch_gdk_binaries.sh

### Description
The fetch_gdk_binaries.sh script require the curl command in order to download files:
https://github.com/Blockstream/green_android/blob/cfa2ecff9ea53dc58ccfb83614961f84f4b4add2/app/fetch_gdk_binaries.sh#L19
https://github.com/Blockstream/green_android/blob/cfa2ecff9ea53dc58ccfb83614961f84f4b4add2/app/fetch_gdk_binaries.sh#L37
using curl or wget, whatever is available is better, alpine by default comes with wget command, thus no need to install additional package in order to build the app with precomopiled binaries.

### Expected behaviour

Running the fetch_gdk_binaries.sh script under alpine works without installing curl as wget used instead, curl can be used if it's available, just not mandatory if wget command alternative is available.