Revert RUSTFLAGS env var leak workaround in build.yaml

[Copilot]

Summary

Reverts the temporary RUSTFLAGS environment variable workaround in the cargo make build step of .github/workflows/build.yaml.

Changes

Removed:

• The $env:RUSTFLAGS = $null line that unset RUSTFLAGS before running cargo make
• The associated comment explaining the workaround
• The shell: pwsh directive (no longer needed with a single-line run)

Restored the step to a simple single-line command:

run: cargo make default +${{ matrix.rust_toolchain }} --locked --profile ${{ matrix.cargo_profile }} --target ${{ matrix.target_triple.name }}

Context

The workaround was introduced in #49 to prevent RUSTFLAGS (specifically -D warnings) from leaking into rust-script compilation of wdk-build (a path dependency via symlink that defeats --cap-lints). The upstream fix is tracked at microsoft/windows-drivers-rs#629.