fix: add Gitlab Host fallback regression (#147)

Author: vinnymac

.changeset/little-geese-rule.md

@@ -0,0 +1,5 @@
+---
+'changesets-gitlab': patch
+---
+
+fix: gitlab host fallback environment variables

src/env.ts

@@ -11,6 +11,10 @@ export const env = {
   ...process.env,
 
   CI_MERGE_REQUEST_IID: +process.env.CI_MERGE_REQUEST_IID!,
+  GITLAB_HOST:
+    process.env.GITLAB_HOST ??
+    process.env.CI_SERVER_URL ??
+    'https://gitlab.com',
   GITLAB_CI_USER_EMAIL:
     process.env.GITLAB_CI_USER_EMAIL || 'gitlab[bot]@users.noreply.gitlab.com',
   GITLAB_COMMENT_TYPE: process.env.GITLAB_COMMENT_TYPE ?? 'discussion',