nsis: Set correct registry view after (un)installing browser integration

* src/nsis/inst-gpgme-browser.nsi, src/nsis/uninst-gpgme.nsi: Set
registry view back to 64 for 64-bit installations.

This fixes the problem that NSIS wrote to the 32-bit registry instead of
the 64-bit registry after installing the browser integration. And this
broke proper update/uninstallation of the old Gpg4win when installing a
newer Gpg4win.

• GnuPG-bug-id: T8038