ci: build python bindings without pip build isolation (#2036)

## Summary

pip provisions `[build-system].requires` into a fresh PEP 517 build
environment on every run. The pinned `torch==2.11.*` can never be
satisfied by the torch already in the CI images (an NVIDIA nightly, e.g.
`2.13.0a0+8145d630e8.nv26.6`), so pip re-downloaded torch plus its CUDA
dependency closure - several GB - on each invocation.

That step normally completes in ~2 min, but stalls for 10-30 min when
the index or network degrades, tripping the stage timeout and aborting
with exit 143. It caused 10 CI failures between Jul 1 and Aug 3 across
`nixl-ci-gpu`, `nixl-ci-non-gpu` and `nixl-ci-dl-gpu`.

CI does not need an isolated build environment, so this builds with
`--no-build-isolation` and provisions the build dependencies explicitly.
**`[build-system].requires` is left untouched**, so the isolated-build
contract for consumers building from source - including `nixl_ep`'s
torch requirement - is unchanged.

## Changes

`.gitlab/test_python.sh` only:

- add `--no-build-isolation` to the `pip install .` invocation
- install the build dependencies with `--upgrade`, in a single branch

## Why `--upgrade` matters

Without it pip treats an already-present version as satisfying the
requirement and leaves it in place. The images ship **meson 1.3.2**, and
that is what the failing runs actually used:

```
Version: 1.3.2
meson setup ... --native-file=.../meson-python-native-file.ini
meson-python: error: Could not map installation path to an equivalent wheel directory: '{prefix}/lib/libcore.so'
```

`contrib/Dockerfile` already installs these same packages with
`--upgrade`, so this brings the CI script in line with it.

The two branches previously diverged - the venv branch installed the
full set while the system-python branch installed only `tomlkit` and
relied on the image for the rest. Both now install the same set, so the
build no longer depends on image contents.

## Verification

`bash -n` passes. The stall is environment-dependent and not locally
reproducible, so confirmation is a green CI run - specifically that the
isolated build environment is no longer provisioned, and that with an
upgraded meson the wheel builds without the `{prefix}/lib` mapping
error.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Build and Installation**
* Build dependencies are now fully installed and upgraded consistently.
* Package builds use preinstalled build dependencies for more
predictable results.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

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Signed-off-by: NirWolfer <nwolfer@nvidia.com>