Update GitHub Action for PRs to use go 1.15.13

This is required due to golang/go#44557 -- go
1.16 deprecates the ioutil package and introduces a new package io in
its place. Go versions below 1.15.10 can fail if a library or dependency
uses the go 1.16 package, e.g. for addlicense:

build github.com/google/addlicense: cannot load io/fs: malformed module path "io/fs": missing dot in first path element

This commit updates to Go 1.15.13, as that is what's used in the current
builder image for the controller

Signed-off-by: Angel Misevski <[email protected]>

.github/workflows/pr.yml

@@ -15,7 +15,7 @@ jobs:
       name: Set up Go 1.x
       uses: actions/setup-go@v2
       with:
-        go-version: 1.13
+        go-version: 1.15.13
     -
       name: Set up Python 3.8
       uses: actions/setup-python@v2