fix(mise): switch sccache to ubi backend to match shim expectations

The mise shim maps the sccache binary to ubi:mozilla/sccache, but the
tool was installed as github:mozilla/sccache. This backend mismatch
causes the shim to fail with 'No version is set for shim: sccache' on
amd64. The github: backend is deprecated in favor of ubi: in newer mise
versions — switching aligns the tool declaration with what the shim
expects.

Also removes the now-unnecessary 'Activate mise environment' CI step.

Author: drew

.github/workflows/checks.yml

@@ -34,15 +34,6 @@ jobs:
       - name: Install tools
         run: mise install
 
-      - name: Activate mise environment
-        run: |
-          # Register sccache globally so the mise shim can resolve it.
-          # Without this, the shim fails with "No version is set for shim".
-          mise use -g github:mozilla/sccache@0.14.0
-          # Resolve RUSTC_WRAPPER to the absolute sccache binary so cargo
-          # invokes it directly, bypassing the shim entirely.
-          echo "RUSTC_WRAPPER=$(mise which sccache)" >> "$GITHUB_ENV"
-
       - name: Cache Rust target and registry
         uses: Swatinem/rust-cache@779680da715d629ac1d338a641029a2f4372abb5 # v2
         with:

mise.toml

@@ -15,7 +15,7 @@ kubectl = "1.35.1"
 uv = "0.10.2"
 protoc = "29.6"
 helm = "4.1.1"
-"github:mozilla/sccache" = "0.14.0"
+"ubi:mozilla/sccache" = "0.14.0"
 
 [env]
 _.path = ["{{config_root}}/scripts/bin"]