Key rollover: fix the .htaccess files built by Buildscr.

Key rollover: fix the .htaccess files built by Buildscr.

The build script generates the .htaccess files that go in each
individual build and redirect generic names like 'putty.tar.gz' to the
real filenames including that build's version number. Those .htaccess
files redirect the corresponding signatures as well, so they need
updating now that we're generating signature files with a different
extension.