Support `.env` files in `sentry.gradle`

[krystofwoldrich]

Description

Currently, envs like SENTRY_DISABLE_AUTO_UPLOAD have to be set before the build process starts.

sentry.gradle could load {root}/.env and {root}/android/.env to make setting envs easier for users.

[krystofwoldrich]

This might actually have a high impact for novice developers.

As to include custom ENVs in Android Studio the Studio needs to be launched from an environment where the variable is set.

[krystofwoldrich]

We could focus only on supporting .env.sentry-build-plugin