Enable golangci-lint (#49)

## Summary:
It would be nice to have some linting beyond `go vet`!  Now we do.  I
started by copying the config from Khan/webapp.  I did remove a couple
of staticcheck checks that I didn't feel were useful.  (Note also that
exportloopref is the replacement for scopelint in newer golangci-lint.)

Included are all the needed lint fixes; most are stylistic but the
changes in the example are a (minor) bugfix.

Fixes #22.
Issue: https://github.com/Khan/genqlient/issues/22

## Test plan:
make check

Author: benjaminjkraft

Reviewers: aberkan, dnerdy, benjaminjkraft, csilvers, MiguelCastillo

Required Reviewers: 

Approved by: aberkan, dnerdy

Checks: ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Test (1.13), ✅ Lint, ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Test (1.13), ✅ Lint

Pull request URL: https://github.com/Khan/genqlient/pull/49

.github/workflows/go.yml

@@ -2,17 +2,17 @@ name: Go
 
 on:
   push:
-    branches: [ main ]
+    branches: [ "*" ]
   pull_request:
     branches: [ main ]
 
 jobs:
   build:
-    name: Build
+    name: Test
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        go: [ '1.13', '1.14', '1.15', '1.16' ]
+        go: [ '1.13', '1.14', '1.15', '1.16', '1.17' ]
 
     steps:
     - name: Set up Go
@@ -30,3 +30,16 @@ jobs:
         GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
       run: |
         go test -cover -v ./...
+
+  lint:
+    name: Lint
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Git checkout
+      uses: actions/checkout@v2
+
+    - name: Run lint
+      uses: golangci/golangci-lint-action@v2
+      with:
+        version: v1.42   # should match go.mod