update actions/checkout to v2 (#73)

.github/workflows/documentation.yaml

@@ -10,7 +10,7 @@ jobs:
     name: Build and publish documentation
     runs-on: macos-latest
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2
 
       - uses: actions/cache@v2
         with:

.github/workflows/install.yml

@@ -16,7 +16,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2
 
       - uses: actions/cache@v2
         with:

.github/workflows/macos.yml

@@ -16,7 +16,7 @@ jobs:
     runs-on: macos-latest
 
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2
 
       - uses: actions/cache@v2
         with:

.github/workflows/standalone.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2
 
       - uses: actions/cache@v2
         with:

.github/workflows/style.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: macos-latest
 
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2
 
       - uses: actions/cache@v2
         with:

.github/workflows/ubuntu.yml

@@ -17,7 +17,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2
 
       - uses: actions/cache@v2
         with:

.github/workflows/windows.yml

@@ -16,7 +16,7 @@ jobs:
     runs-on: windows-latest
 
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2
 
       - uses: actions/cache@v2
         with: