Fix paths in the "Run tests" steps of the `setup-botan` action on Windows

[jorisdral]

In the setup-botan action, when trying to run the tests for the Botan C library on Windows, the tests fail to run because of bad forward/backward slashes. If we want to be able to run the tests on Windows, we'd probably need to upload a fix to the makefile of the Botan C library.

From .github/actions/setup-botan/action.yml:

  # TODO: on Windows, 'make check' will fail to start running the tests because
  # of wrong backwards/forward slashes in paths and calling the test executable
  # the wrong way. More precisely, 'make check' will run:
  #
  # > ./botan-test.exe --data-dir=.\src\tests/data
  #
  # Where it should be running:
  #
  # > ./botan-test --data-dir=./src/tests/data
  #
  # This might be because we are using bash where 'make check' is expecting a
  # different shell, maybe?
  - name: "🛠️ [Windows]: Run tests"
    if: ${{ inputs.run-tests == 'true' && runner.os == 'Windows' }}
    shell: C:/msys64/usr/bin/bash.exe -e '{0}'
    working-directory: ./tmp-botan
    run: |
      make check

[jorisdral]

Going to remove the test steps altogether in #88