launching python subprocess can fail: `rules_python==1.7.0` + `bootstrap_impl=system_python`

# 🐞 bug report

### Affected Rule

`py_binary() / py_test()` that invokes python as subprocess. When using `--@rules_python//python/config_settings:bootstrap_impl=system_python` (default)

`--@rules_python//python/config_settings:bootstrap_impl=script` does not reproduce this issue.

### Is this a regression?

**Did this behavior use to work in the previous version?** Yes

### Description

I have had python executables where they launched a python subprocess as part of them. A basic one for example looked like

```py
# Python code
import sys

# do some preprocess
subprocess.check_output([sys.executable, "-m", "datamodel_code_generator", ...
# do some post process
```

```py
# BUILD code
py_binary(
name = "pydantic_generator",
deps = ["@pypi//:datamodel_code_generator", ...]
)
```

With `rules_python==1.7.0` I get an error that no module named `datamodel_code_generator` found.

## 🔬 Minimal Reproduction

https://github.com/shayanhoshyari/issue-reports/tree/main/rules_python/subprocess_call

## 🔥 Exception or Error

/private/var/tmp/_bazel_hoshyari/5bf493163e144814d5d8244a8cb0233d/execroot/_main/bazel-out/darwin_arm64-fastbuild/bin/test.runfiles/rules_python++python+python_3_13_aarch64-apple-darwin/bin/python3: No module named datamodel_code_generator

## 🌍 Your Environment

**Operating System:**

```
Reproduces on both ubuntu (tried 22.04) MacOS (tried Sonoma)
```

**Output of `bazel version`:**

```
bazel 8.2.1
```

**Rules_python version:**

```
1.7.0
```

**Anything else relevant?**

Seems this is due to https://github.com/bazel-contrib/rules_python/commit/b8e32c454a1158cd78ce4ecaef809b99bef4e5da

1. `1.6.3` and older used `PYTHONPATH` so it propagated in subprocess calls.
2. `1.7.0` starts using the combination
1. [bootstrap stage 1](https://github.com/bazel-contrib/rules_python/blob/411b937484b7a690fad2bacf2d03fec60bed765b/python/private/python_bootstrap_template.txt) that calls
2. [bootstrap stage 2](https://github.com/bazel-contrib/rules_python/blob/main/python/private/stage2_bootstrap_template.py) that adds `iii` to `site.addsitepackage`, then runs main entrypiont with `runpy`.
3. [siteinit.py](https://github.com/bazel-contrib/rules_python/blob/main/python/private/site_init_template.py) that adds the new imports.

The issue is that adding folders via `site.addsitepackage` do not propagate via `subprocess.call`. So when the main script launches a new python, it is not aware of `ii` and hence `iii`.

`bootstrap_impl=script` does not have this issue as `sys.executable` resolves to a different python (`test.venv/bin/python`) that always is aware of `iii` even when `ii` is out of picture.

(I suspect if we launch with `-S` even `bootstrap_impl=script` will stop working)

**Fix ideas**:
1. use same venv python for `system_python` too
2. use [sitecustomize](https://docs.python.org/3/library/site.html) + PYTHONPATH in bootstrap stage 1 instead of `site.addsitepackage` in bootstrap stage 2. This should propagate.

首先比较 python/private/python_bootstrap_template.txt、python/private/stage2_bootstrap_template.py 和 python/private/site_init_template.py，重点关注 system_python 如何为子进程构建导入路径。使用链接的最小复现验证该失败，然后添加回归覆盖，表明 py_binary 或 py_test 子进程可以在 bootstrap_impl=system_python 下导入其声明的依赖，同时保持现有脚本行为不变。

由索引模型根据 Issue 内容生成。