ci/env.sh: Log `CI_BASE_BRANCH` (#33744)

When debugging CI failures it helps when I can see all the variables set
by `env.sh`.

Order environment variables in the output to match the order they are
set a bit better.

`CI_BASE_BRANCH` was not set at all for unknown environment.  Does not
matter much, but it seems consistent to include it in the list next to
all the other variables.

ci/env.sh

@@ -116,6 +116,7 @@ if [[ -n $CI ]]; then
 else
   export CI=
   export CI_BRANCH=
+  export CI_BASE_BRANCH=
   export CI_BUILD_ID=
   export CI_COMMIT=
   export CI_JOB_ID=
@@ -131,10 +132,12 @@ fi
 cat <<EOF
 CI=$CI
 CI_BRANCH=$CI_BRANCH
+CI_BASE_BRANCH=$CI_BASE_BRANCH
 CI_BUILD_ID=$CI_BUILD_ID
 CI_COMMIT=$CI_COMMIT
 CI_JOB_ID=$CI_JOB_ID
-CI_OS_NAME=$CI_OS_NAME
 CI_PULL_REQUEST=$CI_PULL_REQUEST
+CI_OS_NAME=$CI_OS_NAME
+CI_REPO_SLUG=$CI_REPO_SLUG
 CI_TAG=$CI_TAG
 EOF