fix(ci): replace fake goto-bus-stop/setup-zig SHA pin (#19)

## Summary

The pin
`goto-bus-stop/setup-zig@7ab2955eb728f5440978d7b4f723a50dea1f3608` at
`.github/workflows/e2e.yml:42` is fabricated. `gh api
repos/goto-bus-stop/setup-zig/commits/7ab2955... → 422 "No commit found
for SHA"`.

The fake SHA's first 16 hex chars collide with v2.2.0's real SHA
`7ab2955eb728f5440978d5824358023be3a2802d`, but the rest is fabricated —
a partial collision that would slip past visual review.

Replaced with verified v2.2.1 pin
`abea47f85e598557f500fa1fd2ab7464fcb39406`.

## Provenance

- Propagated from `rsr-template-repo`'s `e2e.yml` template (commented
stub, uncommented when concretising here)
- Template fixed upstream: hyperpolymath/rsr-template-repo#81 (merged)
- Discovered while wiring CI for snifs: hyperpolymath/snifs#30
- 3 of 3 fan-out PRs (alongside `odds-and-sods-package-manager#39` and
`proven#93`)

## Test plan

- [ ] The setup-zig line uses `abea47f85e...` (v2.2.1)
- [ ] `gh api
repos/goto-bus-stop/setup-zig/commits/abea47f85e598557f500fa1fd2ab7464fcb39406`
returns 200
- [ ] The e2e workflow's Zig setup step actually resolves on next run

Author: hyperpolymath

.github/workflows/e2e.yml

@@ -39,7 +39,7 @@ jobs:
         uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5  # v4
 
       - name: Install Zig
-        uses: goto-bus-stop/setup-zig@7ab2955eb728f5440978d7b4f723a50dea1f3608  # v2
+        uses: goto-bus-stop/setup-zig@abea47f85e598557f500fa1fd2ab7464fcb39406  # v2.2.1
         with:
           version: 0.15.0