build: remove unnecessary env.sh logic for circleci (#23054)

In the past, when the `checkout_and_rebase` command has
been ported from `angular/angular` to the components repo,
the `env.sh` w/ `env-helper.inc.sh` scripts have been integrated
too. The goal of these scripts is to setup the environment for
CircleCI in a CI-agnostic way. Since the only logic relying on
the `env.sh` file is the `rebase-pr` script, we can just
update the script (located in `.circleci/`) to point directly to the
CircleCI variables.

This is more readable and makes the CI less convoluted. Especially
since the `env.sh` script currently performs duplicate work like
settign up the RBE/GCP token. A separate script is already invoked
for this using the `*setup_bazel_rbe` YAML macro.

Author: devversion

.circleci/config.yml

@@ -157,15 +157,12 @@ commands:
     description: Checkout and rebase the repository
     steps:
       - checkout
-      - run:
-          name: Set up environment
-          environment:
-              CIRCLE_GIT_BASE_REVISION: << pipeline.git.base_revision >>
-              CIRCLE_GIT_REVISION: << pipeline.git.revision >>
-          command: ./.circleci/env.sh
+      # After checkout, rebase on top of target branch.
       - run:
           name: Rebase PR on target branch
-          # After checkout, rebase on top of target branch.
+          environment:
+            CIRCLE_GIT_BASE_REVISION: << pipeline.git.base_revision >>
+            CIRCLE_GIT_REVISION: << pipeline.git.revision >>
           command: |
             if [ -n "$CIRCLE_PR_NUMBER" ]; then
               # User is required for rebase.

.circleci/env-helpers.inc.sh

@@ -184,8 +184,18 @@ function getRefAndShas(sha, owner, name) {
 
 /** Gets the refs and shas for the base and target of the current environment. */
 function getRefsAndShasForChange() {
-  const base = getRefAndShas(process.env['CI_GIT_BASE_REVISION'], process.env['CI_REPO_OWNER'], process.env['CI_REPO_NAME']);
-  const target = getRefAndShas(process.env['CI_GIT_REVISION'], process.env['CI_PR_USERNAME'], process.env['CI_PR_REPONAME']);
+  const base = getRefAndShas(
+    process.env['CIRCLE_GIT_BASE_REVISION'],
+    process.env['CIRCLE_PROJECT_USERNAME'],
+    process.env['CIRCLE_PROJECT_REPONAME']
+  );
+
+  const target = getRefAndShas(
+    process.env['CIRCLE_GIT_REVISION'],
+    process.env['CIRCLE_PR_USERNAME'],
+    process.env['CIRCLE_PR_REPONAME']
+  );
+
   const commonAncestorSha = getCommonAncestorSha(base.sha, target.sha);
   return {
     base,