✨ feat(env): control the isolated build environment location - #1083

## Summary
After #559, there's a few more ways to improve caching and get the seeds
to 100% hit rates:
- Use a stable --env-dir so that the Python compilation units are
cacheable
- Use a stable build-dir so that the fanout builds can fully use the
seed builds
- Raise the MACSX_DEPLOYMENT_TARGET so that the cp311 builds can use the
seed builds

## Current problems
1. cibuildwheel uses a random path dir which causes the Python
compilation units to consistently miss. There hasn't been a great
solution until yesterday (2026-08-27) when pypa/build 1.6.0 shipped:
[#1083 control the isolated build environment location w
--env-dir](pypa/build#1083). This feature will
solve one specific issue: the 51 cache misses in the seed builds due to
the random build path created by cibuildwheel.

2. Using a dynamic build-dir means that each build is contaminated by
the cache_tag: the cache_tag ends up part of some ccache compile calls,
which then aren't reused across builds.

https://github.com/duckdb/duckdb-python/blob/6239d169683d93e67a1d522b5d47c4a60e13918f/pyproject.toml#L84

3. The seed cache is dependent on the MACOSX_DEPLOYMENT_TARGET, set
here:

https://github.com/duckdb/duckdb-python/blob/6239d169683d93e67a1d522b5d47c4a60e13918f/pyproject.toml#L510
but cibuildwheel will autoadjust that to 10.15 for some builds and
ignore your input: [cibuildwheel
MACOSX_DEPLOYMENT_TARGET](https://github.com/pypa/cibuildwheel/blob/9c00cb4f6b517705a3794b22395aedc36257242c/cibuildwheel/platforms/macos.py#L307-L313).
This causes a mismatch between cp311 and other builds, causing cp311 to
miss.

Alternative: If you need to support cp311 on 10.14, then could also move
it up to seed so it generates its own cache.

## Changes
- Use --env-dir and pypa/build 1.6.0 to eliminate the random Python dir.
- Set build-dir to a constant when running cibuildwheel
- Use a deployment target that works for all builds: 10.15.
- add verbose logging to help with future debugging: you need the full
path to see what's changed
Versions
- cibuildwheel to 4.2.0 (latest)
- nanobind < 3.0 (mentioned elsewhere, needed for builds due to a
breaking change)

## Current State Notes
- seeds get about 88% hits. The 51 misses are the Python misses fixed by
the `--env-dir` change above
- The fanouts are around ~50% due to the `{cache_tag}`.
- The cp311 macos x86_64 builds have a 0% cache hit rate due to the
MACOSX_DEPLOYMENT_TARGET

## Impact
- 100% cache hits on seeds vs 88% 
- ~88% cache hits on the fanout vs ~51%
- Reference:
https://github.com/duckdb/duckdb-python/actions/runs/32117618656/job/95671628960
- The forked seeds are more or less the same time: the cache hits don't
have a significant runtime difference, since it's dominated by test time
and the 51 python units are fairly fast.
- The forked fanout/matrix builds are 5-10 minutes faster... the caching
makes a big difference.

## Futures / other optimizations worth considering
- Two tests are about 30% of total runtime: `test_executemany_leak` and
`test_materialized_relation[10000000]`
- Don't install ccache twice on Windows and macosx. This is 30sec-1min. 
- Don't install ccache from EPEL on Linux, use curl & cache it. I saw
this take up to 2 minutes.
- Set `-n auto` or `-n 2` for pytests.. there's a few places where
tmppath fixtures will be needed, but this cuts CI time down
significantly.
- Bump setup-uv and uv-version to latest, and enable uv cache.