nsis: Write to correct filename in first place

contrib/gitian-descriptors/gitian-win.yml

@@ -145,10 +145,7 @@ script: |
     make ${MAKEOPTS} -C src check-security
     make deploy
     make install DESTDIR=${INSTALLPATH}
-    (
-      SETUP_EXE="$(basename "$(echo ./*-setup.exe)")"
-      cp -f "$SETUP_EXE" "${OUTDIR}/${SETUP_EXE/%-setup.exe/-setup-unsigned.exe}"
-    )
+    cp -f --target-directory="${OUTDIR}" ./bitcoin-*-setup-unsigned.exe
     cd installed
     mv ${DISTNAME}/bin/*.dll ${DISTNAME}/lib/
     find . -name "lib*.la" -delete