Fix Windows MinGW CI failure by forcing Bash for GoogleTest install step

[Copilot]

The Building the binaries for Windows Latest MinGW workflow job failed because a POSIX shell conditional was executed under PowerShell on windows-latest. This change aligns shell execution with the script syntax to restore Windows MinGW build continuity.

• Root cause

  • The install GoogleTest step uses Bash syntax:
    • if [ "${{ runner.os }}" = "Windows" ]; then ...
  • On Windows runners, the step was interpreted as PowerShell, causing a parser error.

• Workflow change

  • Updated .github/workflows/multi-platform-build.yml:
    • Added shell: bash to the install GoogleTest step.
  • No build logic or dependency versions were changed.

• Resulting behavior

  • The step now runs with the intended shell across matrix targets, so the OS-conditional branch selection works as written.

- name: install GoogleTest
  if: ${{ steps.cache.output.cache-hit != 'true' }}
  shell: bash
  run: |
    git clone https://github.com/google/googletest.git --branch release-1.12.1
    cd googletest
    if [ "${{ runner.os }}" = "Windows" ]; then
      cmake -Bbuild -DCMAKE_INSTALL_PREFIX="$HOME/.local" -Dgtest_force_shared_crt=1 -G "MinGW Makefiles"
    else
      cmake -Bbuild -DCMAKE_INSTALL_PREFIX="$HOME/.local" -G "Unix Makefiles"
    fi