Enable PHP nightly on Travis CI builds

[dregad]

Fixes #33099

[dregad]

That does not quite do what I expected...

$ phpenv global nightly 2>/dev/null
nightly is not pre-installed; installing
Downloading archive: https://storage.googleapis.com/travis-ci-language-archives/php/binaries/ubuntu/18.04/x86_64/php-nightly.tar.bz2
curl -sSf --retry 5 -o archive.tar.bz2 $archive_url && tar xjf archive.tar.bz2 --directory /
$ phpenv global nightly
[...]
$ php --version
PHP 8.0.27-dev (cli) (built: Dec  7 2022 19:29:53) ( ZTS )

So according to Travis, nigthly = 8.0 and was last built in late 2022...

[dregad]

Tried to specify PHP 8.3 instead, but is not yet available...

$ phpenv global 8.3 2>/dev/null
8.3 is not pre-installed; installing
Downloading archive: https://storage.googleapis.com/travis-ci-language-archives/php/binaries/ubuntu/18.04/x86_64/php-8.3.tar.bz2
$ curl -sSf --retry 5 -o archive.tar.bz2 $archive_url && tar xjf archive.tar.bz2 --directory /
curl: (22) The requested URL returned error: 404 
$ phpenv global 8.3
rbenv: version `8.3' not installed
The command "phpenv global 8.3" failed and exited with 1 during .

[dregad]

I wrote to TravisCI support over a month ago but despite several reminders all I got from them to date, was an acknowledgment of receipt...