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
It is totally fine, and expected to use Git Bash for running Bazel
on Windows. In fact this is the most common setup for Bazel on Windows
and it's unrealistic to run without it.
This allows us to remove the old/legacy Powershell setup from CI
which is also quite flaky sometimes and does not reproduce how
Bazel is used on windows-users dev machines.
PR Close#45431