ci: pin nushell version to 0.112.2 (#2440)

## Summary

- The `version: '*'` specifier in `hustcer/setup-nu@v3` started failing
on macOS arm64 around 2026-04-27 16:48Z with `No release for Nushell
matching version specifier * of default feature`. The action SHA was
unchanged, so this is an upstream regression in the wildcard resolution
path.
- Earlier successful runs (e.g.
[25003041167](https://github.com/max-sixty/worktrunk/actions/runs/25003041167))
resolved `*` to `0.112.2`, so pinning to that version restores the
previous behavior and makes the build reproducible.
- Same pinning philosophy as a985e24 (pre-commit hook revs) — concrete
versions over moving refs.

Fixes the failing `test (macos)` check on #2438 and on `main`.

## Test plan

- [ ] `test (linux)` passes
- [ ] `test (macos)` passes — the previously failing job
- [ ] `test (windows)` passes (nushell step is skipped on Windows;
unaffected)
- [ ] `code-coverage` and `benchmarks` jobs still install nushell
successfully

Co-authored-by: worktrunk-bot <254187624+worktrunk-bot@users.noreply.github.com>

Author: worktrunk-bot

.github/workflows/ci.yaml

@@ -117,7 +117,7 @@ jobs:
       if: runner.os != 'Windows'
       uses: hustcer/setup-nu@v3
       with:
-        version: '*'
+        version: '0.112.2'
 
     - name: Install shells - Windows
       if: runner.os == 'Windows'
@@ -265,7 +265,7 @@ jobs:
     - name: Install nushell
       uses: hustcer/setup-nu@v3
       with:
-        version: '*'
+        version: '0.112.2'
 
     - name: 💰 Rust repo cache
       uses: actions/cache@v5
@@ -301,7 +301,7 @@ jobs:
     - name: Install nushell
       uses: hustcer/setup-nu@v3
       with:
-        version: '*'
+        version: '0.112.2'
 
     # Ensure nothing remains from caching
     - run: cargo llvm-cov clean --workspace