Bump go versions in .github/workflows/go.yml (#1237)

* Bump go versions in .github/workflows/go.yml

These are very out of date and it's causing persistent failures in the
fuzz CI step.

* Remove GOPATH=$GOROOT assignment

go1.16 shows a warning:
  "warning: GOPATH set to GOROOT (/opt/hostedtoolcache/go/1.16.0/x64) has no effect"

.github/workflows/go.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        go: [ '1.14', '1.15' ]
+        go: [ 1.15.x, 1.16.x ]
     steps:
 
     - name: Set up Go
@@ -32,13 +32,12 @@ jobs:
     - name: Set up Go
       uses: actions/setup-go@v2
       with:
-        go-version: '1.14'
+        go-version: 1.16.x
 
     - name: Check out code
       uses: actions/checkout@v2
 
     - name: Fuzz
       run: |
-        export GOPATH=$GOROOT
         make -f Makefile.fuzz get
         make -f Makefile.fuzz