chore(ci): correctly compute `$DIST_TAG` in the `deploy-code` CI job

Previously, the `DIST_TAG` environment variable was failing to be
computed correctly in the `deploy-code` CI job, because it relied on the
non-existent `node` executable. It worked with the default executor
(which includes `node`), but not with the `cloud-sdk` executor used in
`deploy-code`, resulting in the following error:

```sh
./.circleci/env.sh: line 59: node: command not found
DIST_TAG=
```

You can see an example failure in the "Set up environment" step logs in
https://app.circleci.com/pipelines/github/angular/angular.js/
170/workflows/32fcacf9-c89b-4020-b3eb-15debe18bb67/jobs/1793

This commit fixes it by computing `$DIST_TAG` using unix tools (`cat`,
`grep`, `sed`) that _are_ available on the docker images of all
executors.

Closes #17067