AWS CodeBuild无法“获取”软件包

I have an AWS CodeBuild job, defined with the following buildspec file (apologies if indentation isn't reproduced correctly):

version: 0.2
env:
  variables:
      PACKAGE: "github.com/user/package" 
phases:
  install: 
      commands:
          - mkdir -p ${GOPATH}/src/${PACKAGE}
          - cp -a ${CODEBUILD_SRC_DIR}/.  ${GOPATH}/src/${PACKAGE}
          - cd ${GOPATH}/src/${PACKAGE} && go get ./...
build:
     commands:
          - cd ${GOPATH}/src/${PACKAGE} && go build -o ${CODEBUILD_SRC_DIR}/application
post_build:
    commands:
      - aws cloudformation package --template-file template.yml --output-template-file serverless.yml --s3-bucket some-bucket
artifacts:
  files:
    - serverless.yml

This fails in the install phase.

The go application I'm trying to build has several sub-packages and external dependencies. When running "go get ./..." I'm getting

cannot find package github.com/user/package/sub-package in any of:
/usr/local/go/src/github.com/user/package/sub-package(from $GOROOT) /go/src/github.com/user/package/sub-package(from $GOPATH) /codebuild/output/src708017258/src/github.com/user/package/sub-package

When "debugging" (by putting in some echos and listing the contents of the newley created folders) it appears that everything is in the right place and everything should just work.