fix(env): add --system flag to uv pip install for environments without virtualenv

[CodeAgentCN]

Problem

Fixes PrimeIntellect-ai/community-environments#455

When running prime env install <owner>/<env-name> outside a virtualenv, uv pip install fails with:

error: No virtual environment found; run `uv venv` to create an environment, or pass `--system` to install into a non-virtual environment

This happens because uv, unlike pip, requires an explicit flag to install outside a virtualenv.

Solution

Added --system flag to all four uv pip install code paths in the env.py install functions:

1. get_install_command() - used for wheel-based installs
2. _install_single_environment() private env wheel path - used for private environment builds
3. _build_install_command() - used for indexed environment installs
4. install() command private env wheel path - used in the multi-env install flow

This allows prime env install to work in environments without a pre-existing virtualenv. When a virtualenv is already active, --system is a no-op (uv detects the venv and uses it).

Related

• Issue: Fix environments with git dependencies not installing from the hub PrimeIntellect-ai/community-environments#455
• This PR adds the --system flag mentioned in CodeAgentCN's comment