Pin versions of GHA actions

Pin major version because master branch of GHA actions can make breaking
changes.

Author: taiki-e

.github/workflows/ci.yml

@@ -42,7 +42,7 @@ jobs:
          - windows-latest
     runs-on: ${{ matrix.os }}
     steps:
-      - uses: actions/checkout@master
+      - uses: actions/checkout@v2
       - name: Install Rust
         # --no-self-update is necessary because the windows environment cannot self-update rustup.exe.
         run: rustup update ${{ matrix.rust }} --no-self-update && rustup default ${{ matrix.rust }}
@@ -61,7 +61,7 @@ jobs:
           - nightly
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@master
+      - uses: actions/checkout@v2
       - name: Install Rust
         run: rustup update ${{ matrix.rust }} && rustup default ${{ matrix.rust }}
       - name: Add targets
@@ -77,7 +77,7 @@ jobs:
     name: dependencies
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@master
+      - uses: actions/checkout@v2
       - name: Install Rust
         run: rustup update nightly && rustup default nightly
       - name: dependency tree check
@@ -88,7 +88,7 @@ jobs:
     name: rustfmt
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@master
+      - uses: actions/checkout@v2
       - name: Install Rust
         run: rustup update stable && rustup default stable
       - name: rustfmt
@@ -99,7 +99,7 @@ jobs:
     name: clippy
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@master
+      - uses: actions/checkout@v2
       - name: Install Rust
         run: rustup update stable && rustup default stable
       - name: clippy