Improves stability of the Windows Bazel CI job by
installing Bazelisk globally.
Also makes the environment helpers more convenient by
evaluating the variable assignments directly, simplifying
some Bash logic.
PR Close#45431
Previously, the `aio_monitoring_stable` job (which runs tests against
https://angular.io/) was using the tests from the master branch. As a
result, if the master branch included changes in those tests that were
not yet backported to the stable branch (and thus deployed to
https://angular.io/), the tests would fail.
This commit fixes this by using the tests from the stable branch to test
against https://angular.io/.
Fixes#30101
PR Close#30110