ci: clear sccache env on cuda-check so cargo doesn't try to wrap rustc

CI run 255 job 3 (CUDA type-check) fails with:

  error: could not execute process `*** rustc -vV` (never executed)
    Caused by: No such file or directory (os error 2)

The redacted `***` is `sccache`. The ci.yml workflow-level env block
sets `RUSTC_WRAPPER: sccache` because the generic `rust` runner has
sccache installed and routes the cache to caveman.kosherinata.internal.
The new `cuda-check` job runs on `cuda-13.0` (where nvcc lives), and
that runner doesn't carry sccache on PATH — so cargo's first action
(`sccache rustc -vV` to probe the compiler version) fails before
borrow-check even starts.

`build-prerelease.yml`, which uses the same `cuda-13.0` runner for
the actual release neuron builds, deliberately does NOT set
RUSTC_WRAPPER. That's the pattern this commit applies.

Fix: override `RUSTC_WRAPPER` (plus the SCCACHE_* and AWS_* env
locally on the job. We lose caching on the cuda-check job (it's
borrow-check-only and finishes in a couple minutes anyway), but
the gate runs.

The job's purpose — fail fast on `#[cfg(feature = "cuda")]`
borrowck errors that the default-feature gate misses — is what
matters, and that purpose was undermined by the env inheritance.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -105,6 +105,23 @@ jobs:
   cuda-check:
     name: CUDA type-check
     runs-on: cuda-13.0
+    # The workflow-level env sets `RUSTC_WRAPPER: sccache` for the
+    # `rust` runner (where fmt/clippy/test live and sccache is
+    # installed). The `cuda-13.0` runner doesn't have sccache on
+    # PATH, so inheriting the wrapper makes cargo bail with
+    # `could not execute process `sccache rustc -vV` (never executed)`
+    # before borrow-check even starts. Clear it locally. Also clear
+    # SCCACHE_* so cargo doesn't try to contact the cache (the
+    # remote auth headers come from secrets that aren't present on
+    # this runner either). Lose the cache, keep the gate.
+    env:
+      RUSTC_WRAPPER: ""
+      SCCACHE_BUCKET: ""
+      SCCACHE_ENDPOINT: ""
+      SCCACHE_REGION: ""
+      SCCACHE_S3_USE_SSL: ""
+      AWS_ACCESS_KEY_ID: ""
+      AWS_SECRET_ACCESS_KEY: ""
     steps:
       - uses: actions/checkout@v4
       - name: cargo check --features cuda (with retry)