chore: pin third-party GitHub Actions to commit SHAs (#511)

## Summary

Pin all third-party GitHub Actions to full-length commit SHAs to prevent
supply chain attacks.

Addresses findings from the
[`third-party-action-not-pinned-to-commit-sha`](https://github.com/launchdarkly/semgrep-rules/blob/main/github-actions/third-party-action-not-pinned-to-commit-sha.yml)
Semgrep rule.

## Test plan

- [ ] Verify CI passes with pinned action SHAs

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk: CI-only change that pins an existing third-party action to a
specific commit for supply-chain hardening, without altering inputs or
behavior beyond the referenced revision.
> 
> **Overview**
> Pins the Linux `MarkusJx/install-boost` step in
`.github/actions/install-boost/action.yml` from the `v2.4.4` tag to its
full commit SHA, improving supply-chain security while keeping the same
action version.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
56d9a44. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: pkaeding

.github/actions/install-boost/action.yml

@@ -22,7 +22,7 @@ runs:
   steps:
     - name: Install boost using action
       if: runner.os == 'Linux'
-      uses: MarkusJx/install-boost@v2.4.4
+      uses: MarkusJx/install-boost@3039450bb3dd2e8630d1cf10ec39cb1da3054bbd # v2.4.4
       id: boost-action
       with:
         boost_version: 1.81.0