Preserve environment for script subprocesses

[jkramer5103]

What changed

• launch script subprocesses with ProcessBuilder so they inherit Minecraft's environment
• overlay Minescript's configured PYTHONPATH, MINESCRIPT_COMMAND_PATH, and custom command environment values
• add a regression test covering inherited variables, overrides, and values containing =

Root cause

Runtime.exec(command, envp) treats a non-null envp as the subprocess's complete environment. Minescript passed only its custom variables, so standard Windows variables such as LOCALAPPDATA were removed.

The Windows Python Install Manager uses LOCALAPPDATA while scanning unmanaged installations. When Minescript launched the default WindowsApps\\python3.exe alias without that variable, Python Manager tried to construct a path from None and printed:

[WARNING] Failed to read unmanaged installs: expected str, bytes or os.PathLike object, not NoneType

Python Manager catches that exception and continues, which is why scripts still worked while the warning appeared for every invocation.

Fixes #69.

Validation

• ./gradlew --configure-on-demand :common:test — passes
• :common:compileJava — passes as part of the test task
• git diff --check — passes

The repository's unrestricted Gradle configuration currently fails before task execution because ForgeGradle rejects the repository's Gradle 9.2 wrapper. Configuration-on-demand isolates and validates the affected common module.

[jkramer5103]

Currently don't have a windows machine to test this issue, im pretty sure this fixes it, but I asked the person that submitted the issue to test it out, we'll see.

[maxuser0]

See this recent commit about copyright attribution going forward:

8e5e798

Existing sources still have the old copyright notices, but I'll update those over time.

[maxuser0]

Same here.