Add CI environment variables to user-agent

The `npm-in-ci` header has been sent for some time now by
make-fetch-happen, but User-Agent is more reliably logged and respected
by proxies and CDNs, so that's a better place to put this.

lib/config/defaults.js

@@ -201,7 +201,8 @@ Object.defineProperty(exports, 'defaults', {get: function () {
     'user-agent': 'npm/{npm-version} ' +
                     'node/{node-version} ' +
                     '{platform} ' +
-                    '{arch}',
+                    '{arch} ' +
+                    '{ci}',
     'read-only': false,
     'rebuild-bundle': true,
     registry: 'https://registry.npmjs.org/',

lib/npm.js

@@ -281,7 +281,27 @@
         ua = ua.replace(/\{npm-version\}/gi, npm.version)
         ua = ua.replace(/\{platform\}/gi, process.platform)
         ua = ua.replace(/\{arch\}/gi, process.arch)
-        config.set('user-agent', ua)
+
+        // continuous integration platforms
+        const ci = process.env.GERRIT_PROJECT ? 'ci/gerrit'
+          : process.env.GITLAB_CI ? 'ci/gitlab'
+          : process.env.CIRCLECI ? 'ci/circle-ci'
+          : process.env.SEMAPHORE ? 'ci/semaphore'
+          : process.env.DRONE ? 'ci/drone'
+          : process.env.GITHUB_ACTION ? 'ci/github-actions'
+          : process.env.TDDIUM ? 'ci/tddium'
+          : process.env.JENKINS_URL ? 'ci/jenkins'
+          : process.env['bamboo.buildKey'] ? 'ci/bamboo'
+          : process.env.GO_PIPELINE_NAME ? 'ci/gocd'
+          // codeship and a few others
+          : process.env.CI_NAME ? `ci/${process.env.CI_NAME}`
+          // test travis last, since many of these mimic it
+          : process.env.TRAVIS ? 'ci/travis-ci'
+          : process.env.CI === 'true' ? 'ci/custom'
+          : ''
+        ua = ua.replace(/\{ci\}/gi, ci)
+
+        config.set('user-agent', ua.trim())
 
         if (config.get('metrics-registry') == null) {
           config.set('metrics-registry', config.get('registry'))