Update CPM.cmake and cache dependencies in workflows (#70)

* update CPM.cmake to 0.28.0

* cache CPM.cmake dependencies

* create modules directory if it doesn't exist on windows

* experiment with env variable

* experiment 2

* experiment 3

* use env for all workflows

* make cache keys OS dependent

* replace env.GITHUB_ with github.workspace

* format workflow files

* update cache key

* use unique key for each workflow

.github/workflows/standalone.yml

@@ -10,17 +10,23 @@ on:
 
 jobs:
   build:
-
     runs-on: ubuntu-latest
-    
+
     steps:
-    - uses: actions/checkout@v1
-    
-    - name: configure
-      run: cmake -Hstandalone -Bbuild -DCMAKE_BUILD_TYPE=Debug
+      - uses: actions/checkout@v1
 
-    - name: build
-      run: cmake --build build -j4
+      - uses: actions/cache@v2
+        with:
+          path: "**/cpm_modules"
+          key: ${{ github.action }}-cpm-modules-${{ hashFiles('**/CMakeLists.txt', '**/*.cmake') }}
 
-    - name: run
-      run: ./build/Greeter
+      - name: configure
+        env:
+          CPM_SOURCE_CACHE: ${{ github.workspace }}/cpm_modules
+        run: cmake -Sstandalone -Bbuild -DCMAKE_BUILD_TYPE=Debug -DCMAKE_BUILD_TYPE=Debug
+
+      - name: build
+        run: cmake --build build -j4
+
+      - name: run
+        run: ./build/Greeter