[rush] Add experiment to trim RUSH_ env vars forwarded to operation processes - #5994

Rush forwards nearly its entire process environment to operation processes (build/test scripts), including internal RUSH_-prefixed variables that operation scripts have no business depending on. This adds an opt-in experiment to exclude them.

Experiment flag

• New trimRushEnvironmentVariablesForOperations boolean added to IExperimentsJson and experiments.schema.json. Disabled by default; no behavior change unless a repo opts in via experiments.json.

Filtering plugin

• New TrimRushEnvironmentVariablesPlugin taps the existing createEnvironmentForOperation waterfall hook (same extension point as IgnoredParametersPlugin/NodeDiagnosticDirPlugin) and deletes any env var whose name starts with RUSH_ (case-insensitive), e.g.:

// experiments.json
{
  "trimRushEnvironmentVariablesForOperations": true
}

• RUSHSTACK_* and other non-RUSH_ variables are left untouched.

Wiring

• PhasedScriptAction registers the plugin before other env-forwarding plugins only when the experiment is enabled, scoping the trim strictly to operation processes (build/test/etc.), not rushx, event hooks, or custom global commands.

Docs/API

• Updated common/reviews/api/rush-lib.api.md for the new public IExperimentsJson member and added a changelog entry for @microsoft/rush-lib.