github: update checkout action to v2

Checkout v1 has a known flake:
https://github.com/actions/checkout/issues/23#issuecomment-572688577.

For our linter to pass, we need to checkout our full history (default
depth is 1 commit). We could set fetch-depth, but we will eventually
move that depth past the linter's start point commit and need to
implement another fix. Indead, we add an extra step in our linter to
fetch full history so that the linter reference commit is found.

.github/workflows/main.yml

@@ -29,7 +29,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: git checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: go cache
         uses: actions/cache@v1
@@ -70,7 +70,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: git checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: go cache
         uses: actions/cache@v1
@@ -99,7 +99,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: git checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
+
+      - name: Fetch all history for linter
+        run: git fetch --prune --unshallow
 
       - name: go cache
         uses: actions/cache@v1
@@ -134,7 +137,7 @@ jobs:
           - solaris-amd64
     steps:
       - name: git checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: go cache
         uses: actions/cache@v1
@@ -170,7 +173,7 @@ jobs:
           - travis-race
     steps:
       - name: git checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: go cache
         uses: actions/cache@v1
@@ -225,7 +228,7 @@ jobs:
           - neutrino
     steps:
       - name: git checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: go cache
         uses: actions/cache@v1