fix(build): use container pnpm for Alpine Docker builds

Fix "exec format error" when building Alpine ARM64 binaries by
ensuring the container uses its own pnpm installation instead of
the host's x64 binary:
- Override PNPM_HOME to point to container's pnpm location
- Use sh -c to ensure clean environment execution

This prevents architecture mismatch errors when building cross-platform.

Author: jdalton

.github/workflows/build-smol.yml

@@ -428,8 +428,9 @@ jobs:
             --platform linux/${{ matrix.arch == 'x64' && 'amd64' || 'arm64' }} \
             -v $PWD:/workspace \
             -w /workspace \
+            -e PNPM_HOME=/usr/local/bin \
             smol-alpine-builder:${{ matrix.arch }} \
-            pnpm --filter @socketbin/node-smol-builder run build
+            sh -c 'pnpm --filter @socketbin/node-smol-builder run build'
 
       - name: Verify smol binary
         if: steps.check-platform.outputs.should-run == 'true'