build: Use PACKAGE_TARNAME and new bin names in NSIS script.

Replaces the hardcoded string "bitcoin" with the autoconf variable PACKAGE_TARNAME; fixes #7265.

Places where I chose not to replace:

1. bitcoin.ico wasn't replaced because it doesn't seem to be relevant to the build system and its filename never affects the end user.
2. InstallDir wasn't replaced because the current text has an uppercase B, and I'm not sure of a good way to capitalize the result of PACKAGE_TARNAME.
3. A comment in the Main Installer section wasn't replaced because comments don't ever face the end user.
4. The registry value "URL:Bitcoin" wasn't replaced for the same reason as InstallDir.
5. Startup shortcut wasn't replaced for the same reason as InstallDir.

All other appearances of "bitcoin" were replaced with PACKAGE_TARNAME, except for the bin names, which were instead replaced with the new bin name autoconf variables.