fix(ci): SHA-pin actions/cache in abi-drift workflow (standards#92 follow-up) (#104)

## Summary

The abi-drift workflow shipped via #98 with `actions/cache@v4` (tag),
which the boj-server policy rejects:

```
##[error]The action actions/cache@v4 is not allowed in hyperpolymath/boj-server
because all actions must be pinned to a full-length commit SHA.
```

Every run since landing has failed at "Set up job" on this. Fixing the
one offending line.

```diff
- uses: actions/cache@v4
+ uses: actions/cache@d4323d4 # v4.2.2
```

The two other actions in the workflow (`actions/checkout`,
`dtolnay/rust-toolchain`) were already SHA-pinned.

## Test plan

- [x] grep confirms this was the only unpinned action in `abi-drift.yml`
- [ ] CI: next run should pass "Set up job" and proceed to the install +
verify steps (or surface a different failure, which is information
either way)

Refs hyperpolymath/standards#92

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

.github/workflows/abi-drift.yml

@@ -50,7 +50,7 @@ jobs:
           toolchain: stable
 
       - name: Cache cargo bin (iseriser install)
-        uses: actions/cache@v4
+        uses: actions/cache@d4323d4df104b026a6aa633fdb11d772146be0bf # v4.2.2
         with:
           path: |
             ~/.cargo/bin/iseriser