fix: forward termination signal

[baszalmstra]

Description

pixi exec and pixi run previously hid signal terminations from the user. When a child process was killed by a signal such as SIGSEGV, pixi silently exited (with code 1 in the case of pixi exec) and no "Segmentation fault" message reached the terminal. This change makes both commands surface signal terminations the same way the shell would if the command had been run directly.

Before:

$ pixi exec python -c "import ctypes; ctypes.string_at(0)"
$ echo $?
1

After:

$ pixi exec python -c "import ctypes; ctypes.string_at(0)"
Segmentation fault (core dumped)
$ echo $?
139

Fixes #6166

How Has This Been Tested?

Built pixi from this branch and ran the command from the issue against both pixi exec and pixi run (inline and via a task). All three cases now print Segmentation fault and exit with 139, matching plain python3. Verified that normal exits (0, non-zero like 42) still propagate correctly.

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.

Tools: Claude

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added sufficient tests to cover my changes.