After this PR, my experimental rust-based extension template (https://github.com/samansmink/extension-template-rs) should be one step closer to reality.

This requires some tweaks to the workflows because things like running tests and platform detection are different from the C++ api based extensions that do have a duckdb submodule in their repository.

Changes:

• you can now disable running the tests by setting the SKIP_TESTS variable
• tests on linux are now run twice: once inside docker, once outside
• there's a new configure_ci step which is called once inside docker, once outside
• extensions can detect which one is being called and set SKIP_TESTS for either to work around any issues
• test_* targets no longer automatically build too, this avoids double building that would happen
• custom-toolchain-script.sh was removed again in favor of more flexible make configure_ci