security: remove COREPACK_INTEGRITY_KEYS from ci env

[elevatebart]

In the gradle build we use the org.siouan.frontend-jdk21 to build the frontend.
This plugin uses corepack to install the proper package manager (npm, pnpm or yarn).
Since we use the one included in nodejs, NPM. We don not need corepack.

Sometimes, the npm registry integrity keys get cycled and we need a new corepack version installed to be able to install npm.

This made the build fail here:

https://github.com/kestra-io/kestra/actions/runs/13049445779/job/36406226636?pr=7077

With this additional env, we dismiss the integrity check.

I hope to re-enable the integrity check ASAP.