Enable ext/curl to speed up Composer installs

[sebastianbergmann]

another thing I noticed in your CI output was this warning: "Composer is operating significantly slower than normal because you do not have the PHP curl extension enabled." - if you don't have a good reason to have curl disabled there I'd say it'd make sense to enable it.

Originally posted by @Seldaek in composer/composer#12360 (comment)

[sebastianbergmann]

phpunit/phpunit actually has ext/curl in the respective GitHub Actions job(s), but phpunit/php-code-coverage (and possibly other dependencies) does not.