[phast3] Changed directory seps

Author: scharlton2

.github/workflows/cmake-multi-platform.yml

@@ -81,7 +81,7 @@ jobs:
 
     - name: Set CONDA_PREFIX
       shell: bash
-      run: echo "CONDA_PREFIX=${{ env.MAMBA_ROOT_PREFIX }}\envs\phast-dev" >> "$GITHUB_ENV"
+      run: echo "CONDA_PREFIX=${{ env.MAMBA_ROOT_PREFIX }}/envs/phast-dev" >> "$GITHUB_ENV"
 
     - name: Configure CMake
       # Configure CMake in a 'build' subdirectory. `CMAKE_BUILD_TYPE` is only required if you are using a single-configuration generator such as make.