fix: "Lint PR Title" job cache validation error (#2722)

## Problem

The "Lint PR Title" job in the PR workflow was consistently failing on
first run with the following error:

> Error: Path Validation Error: Path(s) specified in the action for
caching do(es) not exist, hence no cache is being saved.

After a rerun, the job would usually succeed, but this was annoying and
required manual intervention.

## Root Cause

The `lint-title` job uses the `microsoft-setup-toolchain` action, which
has `cache-npm-dependencies: yarn` enabled by default. This causes the
underlying `actions/setup-node` action to attempt to cache yarn
directories. However, the `lint-title` job never runs `yarn install` -
it only executes `npx @rnx-kit/[email protected]` to validate the PR
title.

Since the cache paths don't exist when the job runs for the first time
(before dependencies are installed), the cache action fails with a
validation error.

## Solution

Disabled caching for the `lint-title` job by setting
`cache-npm-dependencies: ''` in the setup-toolchain step. This is
appropriate because:

1. The job doesn't install any dependencies - it only uses `npx` to run
a single package
2. Caching provides no benefit for this job
3. This eliminates the cache path validation error

## Testing

- Verified the YAML syntax is valid
- The change is minimal and only affects the `lint-title` job
- Other jobs that need caching remain unchanged

Fixes #2403

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<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>"Lint PR Title" PR Job always fails</issue_title>
> <issue_description>The job always fails with the following error:
> 
> > Error: Path Validation Error: Path(s) specified in the action for
caching do(es) not exist, hence no cache is being saved.
> 
> After a rerun it usually succeeds. But it's quite annoying, and would
be nice to fix. </issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>
Fixes #2721

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---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: Saadnajmi <[email protected]>

Author: Copilot

.github/workflows/microsoft-pr.yml

@@ -24,6 +24,7 @@ jobs:
         uses: ./.github/actions/microsoft-setup-toolchain
         with:
           node-version: '22'
+          cache-npm-dependencies: ''
       # We lint the PR title instead of the commit message to avoid script injection attacks.
       # Using environment variables prevents potential security vulnerabilities as described in:
       # https://docs.github.com/en/actions/security-for-github-actions/security-guides/security-hardening-for-github-actions#example-of-a-script-injection-attack