This fixes CI script tests that accidentally run helper scripts with the
wrong Python interpreter.

Some tests call `run_script(..., env={...})` to pass test-specific
environment variables. `subprocess.run` treats `env` as a full
environment replacement, so the subprocess lost the current `PATH`.
Since these scripts use a shebang like:

```python
#!/usr/bin/env python3
```

they could fall back to the system Python instead of the active test
Python.

In the wheel test environment this caused the CI helper scripts to run
under Python 3.8, even though TVM requires Python >=3.10. Importing
`ci/scripts/jenkins/git_utils.py` then failed on PEP 604 annotations
such as:

```python
Any | None
tuple[str, str] | None
```

with:

```text
TypeError: unsupported operand type(s) for |: '_SpecialForm' and 'NoneType'
```

This patch updates `tests/python/ci/test_utils.py::run_script` to merge
test-provided environment variables into `os.environ` instead of
replacing the entire environment. This preserves the active Python/PATH
while still allowing each test to override or add variables.

(cherry picked from commit 795cd2d)

This fixes CI script tests that accidentally run helper scripts with the
wrong Python interpreter.

Some tests call `run_script(..., env={...})` to pass test-specific
environment variables. `subprocess.run` treats `env` as a full
environment replacement, so the subprocess lost the current `PATH`.
Since these scripts use a shebang like:

```python
#!/usr/bin/env python3
```

they could fall back to the system Python instead of the active test
Python.

In the wheel test environment this caused the CI helper scripts to run
under Python 3.8, even though TVM requires Python >=3.10. Importing
`ci/scripts/jenkins/git_utils.py` then failed on PEP 604 annotations
such as:

```python
Any | None
tuple[str, str] | None
```

with:

```text
TypeError: unsupported operand type(s) for |: '_SpecialForm' and 'NoneType'
```

This patch updates `tests/python/ci/test_utils.py::run_script` to merge
test-provided environment variables into `os.environ` instead of
replacing the entire environment. This preserves the active Python/PATH
while still allowing each test to override or add variables.

(cherry picked from commit 795cd2d)