Install hhvm-nightly before sudo is disabled

Ideally, phpenv would be able to do some magic so that it can install
hhvm-nightly without `sudo`, but until then, we will hack our way
here.
use hhvm? and nightly?
Install Firefox add-on under `$HOME` without sudo

And modify `$PATH` so that `firefox` can be found

We can't use `ENV['PATH']` or `ENV['HOME']` to set values,
since the build script is compiled by the build API, which
has different values.
Clean up Firefox addon
Fix announcement [skip ci]