Add getenv builtin to os plugin

[badcount]

What changed?
A new Starlark builtin os.getenv(key, default=None) that lets workflow code read a single key from the environ dict, without having to grab the full os.environ map and index
into it.

Why?
The environ property already exposed the whole dict, but callers wanting a single key had to write os.environ.get("FOO", default) or os.environ["FOO"]. os.getenv("FOO",
default) mirrors Python's os.getenv and is more ergonomic for the common single-key case. The environ data comes from PipelineRun spec.input.environ.

Surface impact

• Public Starlark API: new function os.getenv.
• Go API: unchanged exports — _getenv and environ are unexported; only the Module and the Plugin registration are public.
• Behavioral compatibility: purely additive; os.environ still works the same way.

How did you test it?
Unit test and end to end run

Potential risks

Release notes

Documentation Changes

[gitar-bot]

Code Review ✅ Approved

Adds the os.getenv builtin to the os plugin to provide a more ergonomic way to access environment variables. No issues found.

Rules ❌ No requirements met

Repository Rules

❌ PR Description Quality Standards: The [How did you test it?] section is vague, and [Potential risks], [Release notes], and [Documentation Changes] sections are empty without even 'N/A' placeholders.

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