guix: Use "win64" for Windows artifacts consistently

Author: hebasto

contrib/guix/libexec/build.sh

@@ -439,8 +439,8 @@ mkdir -p "$DISTSRC"
                 find . -print0 \
                     | sort --zero-terminated \
                     | tar --create --no-recursion --mode='u+rw,go+r-w,a+X' --null --files-from=- \
-                    | gzip -9n > "${OUTDIR}/${DISTNAME}-win-unsigned.tar.gz" \
-                    || ( rm -f "${OUTDIR}/${DISTNAME}-win-unsigned.tar.gz" && exit 1 )
+                    | gzip -9n > "${OUTDIR}/${DISTNAME}-win64-unsigned.tar.gz" \
+                    || ( rm -f "${OUTDIR}/${DISTNAME}-win64-unsigned.tar.gz" && exit 1 )
             )
             ;;
     esac