perf(ci): add build parallelism env vars to Windows CI setup

Export CARGO_BUILD_JOBS, RUSTFLAGS (-Zthreads), and
CARGO_PROFILE_RELEASE_CODEGEN_UNITS to match build.ps1 settings.
Without these, codegen-units=1 from Cargo.toml serializes LLVM codegen
to a single thread, dropping CPU utilization to 20-30% on the 8-core
i7-9700K runner during mt-tauri compilation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: pythoninthegrass

.github/actions/setup-tauri-build/action.yml

@@ -65,6 +65,13 @@ runs:
         echo "RUSTUP_TOOLCHAIN=nightly-2026-02-09" >> $env:GITHUB_ENV
         rustup toolchain install nightly-2026-02-09 --no-self-update --target x86_64-pc-windows-msvc
         rustup default nightly-2026-02-09
+        # Parallelism: match build.ps1 settings
+        $cpus = [Environment]::ProcessorCount
+        $buildJobs = [Math]::Max(1, $cpus - 2)
+        $rustThreads = [Math]::Max(1, [Math]::Round($cpus * 0.8))
+        echo "CARGO_BUILD_JOBS=$buildJobs" >> $env:GITHUB_ENV
+        echo "RUSTFLAGS=-Zthreads=$rustThreads" >> $env:GITHUB_ENV
+        echo "CARGO_PROFILE_RELEASE_CODEGEN_UNITS=$rustThreads" >> $env:GITHUB_ENV
         rustc --version
         cargo --version