fix(ci): unset RUSTC_WRAPPER during sccache install

The workflow-level env set RUSTC_WRAPPER=sccache for every step,
including the install step itself. cargo install sccache then
tried to invoke `sccache rustc -vV` to detect the toolchain before
sccache existed on PATH, failing with "No such file or directory".
Override RUSTC_WRAPPER to empty on the install step so cargo uses
rustc directly; subsequent steps still inherit the wrapper.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -24,6 +24,8 @@ jobs:
       - uses: actions/checkout@v4
 
       - name: Ensure sccache with S3 support
+        env:
+          RUSTC_WRAPPER: ""
         run: |
           if sccache --version 2>/dev/null && sccache --show-stats 2>/dev/null; then
             echo "sccache with S3 support already installed"