Handle also main as DUCKDB_GIT_VERSION for C-API extensions

[carlopi]

Currently using DUCKDB_GIT_VERSION=main fails for C-API extensions since main is not a valid version.

Default to latest then when installing the Pyhton-based package.

Tests of this working are at https://github.com/carlopi/extension-ci-tools/actions/runs/12633104809/job/35198062606, where a second problem is shown connected to hardcoding MINIMUM_DUCDKB_VERSION, to be solved independently.

[samansmink]

Not sure how useful this is by itself due to the fact that you will manually need to figure out which duckdb version is latest unstable on PyPI. Does not hurt either I guess

[carlopi]

Not sure how useful this is by itself due to the fact that you will manually need to figure out which duckdb version is latest unstable on PyPI. Does not hurt either I guess

I think as long as latest duckdb is "about v1.2.0 or above", it should be fine to use main to test this right?
I bumped against this while running CI using main, works already when building also duckdb, with this in works also for C-API extensions (they will get a recent duckdb build).

We might consider fixing it to use latest stable, but currently given v1.2.0-compatible CAPI are not released yet, only recent nightly can be used.