build: set Unicode true for NSIS installer

Now that we are using Focal for gitian builds, and have NSIS 3.0+
available (also in Guix), we can create installers that support unicode.

Unicode is only becoming the default beginning with the 3.07 release,
https://nsis.sourceforge.io/Docs/AppendixF.html#v3.07-cl,
so we need to set this attribute to get support.

Should close: #13817

Co-authored-by: Chun Kuan Lee <ken2812221@gmail.com>

share/setup.nsi.in

@@ -3,6 +3,7 @@ Name "@PACKAGE_NAME@ (64-bit)"
 RequestExecutionLevel highest
 SetCompressor /SOLID lzma
 SetDateSave off
+Unicode true
 
 # Uncomment these lines when investigating reproducibility errors
 #SetCompress off