[build-utils] Fix getPrefixedEnvVars() to handle VERCEL_BRANCH_URL (#10315)

- Fixes [NEXT-1500](https://linear.app/vercel/issue/NEXT-1500)
- See
[documentation](https://vercel.com/docs/concepts/projects/environment-variables/system-environment-variables#framework-environment-variables)

packages/build-utils/test/unit.get-prefixed-env-vars.test.ts

@@ -13,6 +13,8 @@ describe('Test `getPrefixedEnvVars()`', () => {
         envs: {
           VERCEL: '1',
           VERCEL_URL: 'example.vercel.sh',
+          VERCEL_ENV: 'production',
+          VERCEL_BRANCH_URL: 'example-git-main-acme.vercel.app',
           USER_ENV_VAR_NOT_VERCEL: 'example.com',
           VERCEL_ARTIFACTS_TOKEN: 'abc123',
           FOO: 'bar',
@@ -20,6 +22,8 @@ describe('Test `getPrefixedEnvVars()`', () => {
       },
       want: {
         NEXT_PUBLIC_VERCEL_URL: 'example.vercel.sh',
+        NEXT_PUBLIC_VERCEL_ENV: 'production',
+        NEXT_PUBLIC_VERCEL_BRANCH_URL: 'example-git-main-acme.vercel.app',
         TURBO_CI_VENDOR_ENV_KEY: 'NEXT_PUBLIC_VERCEL_',
       },
     },