make cache keys OS dependent

.github/workflows/documentation.yaml

@@ -15,7 +15,7 @@ jobs:
       - uses: actions/cache@v2
         with:
           path: '**/cpm_modules'
-          key: cpm-${{ hashFiles('**/CMakeLists.txt', '**/*.cmake') }}
+          key: ${{ runner.os }}-cpm-${{ hashFiles('**/CMakeLists.txt', '**/*.cmake') }}
 
       - name: Install dependencies
         run: |

.github/workflows/install.yml

@@ -22,7 +22,7 @@ jobs:
       - uses: actions/cache@v2
         with:
           path: '**/cpm_modules'
-          key: cpm-${{ hashFiles('**/CMakeLists.txt', '**/*.cmake') }}
+          key: ${{ runner.os }}-cpm-${{ hashFiles('**/CMakeLists.txt', '**/*.cmake') }}
 
       - name: build and install library
         run: |

.github/workflows/macos.yml

@@ -22,7 +22,7 @@ jobs:
       - uses: actions/cache@v2
         with:
           path: '**/cpm_modules'
-          key: cpm-${{ hashFiles('**/CMakeLists.txt', '**/*.cmake') }}
+          key: ${{ runner.os }}-cpm-${{ hashFiles('**/CMakeLists.txt', '**/*.cmake') }}
 
       - name: configure
         env:

.github/workflows/standalone.yml

@@ -19,7 +19,7 @@ jobs:
       - uses: actions/cache@v2
         with:
           path: '**/cpm_modules'
-          key: cpm-${{ hashFiles('**/CMakeLists.txt', '**/*.cmake') }}
+          key: ${{ runner.os }}-cpm-${{ hashFiles('**/CMakeLists.txt', '**/*.cmake') }}
 
       - name: configure
         env:

.github/workflows/style.yml

@@ -19,7 +19,7 @@ jobs:
       - uses: actions/cache@v2
         with:
           path: '**/cpm_modules'
-          key: cpm-${{ hashFiles('**/CMakeLists.txt', '**/*.cmake') }}
+          key: ${{ runner.os }}-cpm-${{ hashFiles('**/CMakeLists.txt', '**/*.cmake') }}
 
       - name: Install format dependencies
         run: |

.github/workflows/ubuntu.yml

@@ -23,7 +23,7 @@ jobs:
       - uses: actions/cache@v2
         with:
           path: '**/cpm_modules'
-          key: cpm-${{ hashFiles('**/CMakeLists.txt', '**/*.cmake') }}
+          key: ${{ runner.os }}-cpm-${{ hashFiles('**/CMakeLists.txt', '**/*.cmake') }}
 
       - name: configure
         env:

.github/workflows/windows.yml

@@ -22,7 +22,7 @@ jobs:
       - uses: actions/cache@v2
         with:
           path: '**/cpm_modules'
-          key: cpm-${{ hashFiles('**/CMakeLists.txt', '**/*.cmake') }}
+          key: ${{ runner.os }}-cpm-${{ hashFiles('**/CMakeLists.txt', '**/*.cmake') }}
 
       - name: configure
         env: