ci: fix update workflows creating duplicate PRs (#26433)

## Summary
- Fixed all `update-*.yml` workflows that were creating duplicate PRs
every week

## Problem
The update workflows (libarchive, zstd, cares, etc.) were using `${{
github.run_number }}` in the branch name, e.g.:
```yaml
branch: deps/update-libarchive-${{ github.run_number }}
```

This caused a new unique branch to be created on every workflow run, so
the `peter-evans/create-pull-request` action couldn't detect existing
PRs and would create duplicates.

**Evidence:** There are currently 8+ open duplicate PRs for libarchive
alone:
- #26432 deps: update libarchive to v3.8.5 (deps/update-libarchive-56)
- #26209 deps: update libarchive to v3.8.5 (deps/update-libarchive-55)
- #25955 deps: update libarchive to v3.8.5 (deps/update-libarchive-54)
- etc.

## Solution
Changed all workflows to use static branch names, e.g.:
```yaml
branch: deps/update-libarchive
```

This allows the action to:
1. Detect if an existing branch/PR already exists
2. Update the existing PR with new changes instead of creating a new one
3. Properly use `delete-branch: true` when the PR is merged

## Files Changed
- `.github/workflows/update-cares.yml`
- `.github/workflows/update-hdrhistogram.yml`
- `.github/workflows/update-highway.yml`
- `.github/workflows/update-libarchive.yml`
- `.github/workflows/update-libdeflate.yml`
- `.github/workflows/update-lolhtml.yml`
- `.github/workflows/update-lshpack.yml`
- `.github/workflows/update-root-certs.yml`
- `.github/workflows/update-sqlite3.yml`
- `.github/workflows/update-vendor.yml`
- `.github/workflows/update-zstd.yml`

## Test plan
- [x] Verified the change is syntactically correct
- [ ] Wait for next scheduled run of any workflow to verify it updates
existing PR instead of creating a new one

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

.github/workflows/update-lolhtml.yml

@@ -100,7 +100,7 @@ jobs:
           commit-message: "deps: update lolhtml to ${{ steps.check-version.outputs.tag }} (${{ steps.check-version.outputs.latest }})"
           title: "deps: update lolhtml to ${{ steps.check-version.outputs.tag }}"
           delete-branch: true
-          branch: deps/update-lolhtml-${{ github.run_number }}
+          branch: deps/update-lolhtml
           body: |
             ## What does this PR do?