fix(gomod): pass GOPROXY

Closes #4071

lib/manager/gomod/artifacts.js

@@ -39,7 +39,8 @@ async function updateArtifacts(
     }
     await fs.outputFile(localGoModFileName, massagedGoMod);
     const localGoSumFileName = upath.join(config.localDir, sumFileName);
-    const env = getChildProcessEnv(['GOPATH']);
+    const customEnv = ['GOPATH', 'GOPROXY'];
+    const env = getChildProcessEnv(customEnv);
     const startTime = process.hrtime();
     let cmd;
     if (config.binarySource === 'docker') {
@@ -47,7 +48,7 @@ async function updateArtifacts(
       cmd = `docker run --rm `;
       const volumes = [config.localDir, process.env.GOPATH];
       cmd += volumes.map(v => `-v ${v}:${v} `).join('');
-      const envVars = ['GOPATH'];
+      const envVars = customEnv;
       cmd += envVars.map(e => `-e ${e} `).join('');
       cmd += '-e CGO_ENABLED=0 ';
       cmd += `-w ${cwd} `;