fix: filter npm-incompatible .npmrc properties in RushVersionSelector

When the Rush version selector uses npm to install a different version of
rush-lib, the .npmrc from the repo is synced but pnpm-specific properties
(like hoist, publish-branch, email, etc.) were not being filtered out.
This caused npm to emit "Unknown env config" and "Unknown project config"
warnings.

Add filterNpmIncompatibleProperties: true to the installPackageInDirectoryAsync
call in RushVersionSelector, matching the pattern already used in
InstallHelpers.ensureLocalPackageManagerAsync().

Fixes #5704

Co-authored-by: iclanton <5010588+iclanton@users.noreply.github.com>
Agent-Logs-Url: https://github.com/microsoft/rushstack/sessions/e3e81359-dd04-4186-979b-7a62e0a663e3

Author: Copilot

apps/rush/src/RushVersionSelector.ts

@@ -62,7 +62,10 @@ export class RushVersionSelector {
           // different implementations of the same version of the same package.
           // This was needed for: https://github.com/microsoft/rushstack/issues/691
           commonRushConfigFolder: configuration ? configuration.commonRushConfigFolder : undefined,
-          suppressOutput: true
+          suppressOutput: true,
+          // Filter out npm-incompatible properties (e.g. pnpm-specific settings) from .npmrc
+          // since this installation always uses npm regardless of the repo's package manager.
+          filterNpmIncompatibleProperties: true
         });
 
         console.log(`Successfully installed Rush version ${version} in ${expectedRushPath}.`);

common/changes/@microsoft/rush/fix-rush-pnpm-npm-config-warnings_2026-03-23-18-16.json

@@ -2,7 +2,7 @@
   "changes": [
     {
       "packageName": "@microsoft/rush",
-      "comment": "Fix \"Unknown env config\" warnings from npm during rush-pnpm operations by passing pnpm configuration via CLI args instead of NPM_CONFIG_* environment variables.",
+      "comment": "Filter npm-incompatible properties from .npmrc when installing rush-lib via npm, to eliminate spurious \"Unknown env config\" and \"Unknown project config\" warnings.",
       "type": "none"
     }
   ],