ci: improve stability of windows bazel CI job (#45431)

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

.circleci/env-helpers.inc.sh

@@ -31,7 +31,10 @@ function setSecretVar() {
   local -r originalShellOptions=$(set +o);
   set +x -eu -o pipefail;
 
-  echo "export $1=\"${2:-}\";" >> $BASH_ENV;
+  local assignmentStatement="export $1=\"${2:-}\";"
+
+  echo "${assignmentStatement}" >> $BASH_ENV;
+  eval "${assignmentStatement}"
 
   # Restore original shell options.
   eval "$originalShellOptions";