title: Why is this Go build failing on Heroku? It builds locally with no issue
body: <p>Is anyone able to tell me why this build has failed on heroku?</p>
  <p>I've tried finding substitute packages and tidying the mod file, even deleting it and redoing it.</p>
  <p>here is my build log</p>
  <pre><code>    Overview Resources Deploy Metrics Activity Access Settings
  Activity Feed  Build LogID f8e9d5ce-0d8f-4e8e-8b2a-661c4d1f68f7
  -----&gt; Building on the Heroku-20 stack
  -----&gt; Go app detected
  -----&gt; Fetching stdlib.sh.v8... done
  -----&gt; 
         Detected go modules via go.mod
  -----&gt; 
         Detected Module Name: github.com/xxxxxx/xxxxxx-xxxxxx
  -----&gt; 
   !!    The go.mod file for this project does not specify a Go version
   !!    
   !!    Defaulting to go1.12.17
   !!    
   !!    For more details see: https://devcenter.heroku.com/articles/go-apps-with-modules#build-configuration
   !!    
  -----&gt; Using go1.12.17
  -----&gt; Determining packages to install
         
         Detected the following main packages to install:
              github.com/xxxxxx/xxxxxx-xxxxxx
         
  -----&gt; Running: go install -v -tags heroku github.com/xxxxxx/xxxxxx-companion 
  github.com/vektah/gqlparser/v2/ast
  github.com/agnivade/levenshtein
  github.com/mitchellh/mapstructure
  github.com/vektah/gqlparser/v2/gqlerror
  github.com/99designs/gqlgen/graphql
  # github.com/99designs/gqlgen/graphql
  ../codon/tmp/cache/go-path/pkg/mod/github.com/99designs/gqlgen@v0.13.0/graphql/error.go:21:5: undefined: errors.As
  github.com/99designs/gqlgen/graphql/errcode
  github.com/vektah/gqlparser/v2/lexer
  github.com/hashicorp/golang-lru/simplelru
  github.com/vektah/gqlparser/v2/parser
  github.com/hashicorp/golang-lru
  github.com/gorilla/websocket
  github.com/vektah/gqlparser/v2/validator
  github.com/99designs/gqlgen/graphql/playground
  github.com/xxxxxx/xxxxxx-xxxxxx/graph/model
  github.com/xxxxxx/xxxxxx-xxxxxx/database
  github.com/99designs/gqlgen/graphql/introspection
  github.com/vektah/gqlparser/v2/validator/rules
  github.com/go-sql-driver/mysql
  github.com/vektah/gqlparser/v2
   !     Push rejected, failed to compile Go app.
   !     Push failed
  </code></pre>
  <p>Here is my go.mod file:</p>
  <pre><code>module github.com/xxxxxx/xxxxxx-xxxxxx
  
  go 1.15
  
  require (
      github.com/99designs/gqlgen v0.13.0
      github.com/go-sql-driver/mysql v1.5.0
      github.com/hashicorp/golang-lru v0.5.1 // indirect
      github.com/mitchellh/mapstructure v0.0.0-20180220230111-00c29f56e238 // indirect
      github.com/stretchr/testify v1.5.1 // indirect
      github.com/vektah/gqlparser/v2 v2.1.0
      gopkg.in/yaml.v2 v2.2.8 // indirect
  )
  </code></pre>
  <p>I'm at a complete loss, I suspect it has to do with an incorrect version, but the build log doesn't give me much to go on. How can I diagnose this kind of issue in the future?</p>
  <p>Thank you.</p>