ci(docs-infra): use the tests from the stable branch in aio_monitoring_stable CircleCI job (#30110)

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

.circleci/env-helpers.inc.sh

@@ -36,3 +36,38 @@ function setSecretVar() {
   # Restore original shell options.
   eval "$originalShellOptions";
 }
+
+
+# Create a function to set an environment variable, when called.
+#
+# Use this function for creating setter for public environment variables that require expensive or
+# time-consuming computaions and may not be needed. When needed, you can call this function to set
+# the environment variable (which will be available through `$BASH_ENV` from that point onwards).
+#
+# Arguments:
+# - `<name>`: The name of the environment variable. The generated setter function will be
+#   `setPublicVar_<name>`.
+# - `<code>`: The code to run to compute the value for the variable. Since this code should be
+#   executed lazily, it must be properly escaped. For example:
+#   ```sh
+#   # DO NOT do this:
+#   createPublicVarSetter MY_VAR "$(whoami)";  # `whoami` will be evaluated eagerly
+#
+#   # DO this isntead:
+#   createPublicVarSetter MY_VAR "\$(whoami)";  # `whoami` will NOT be evaluated eagerly
+#   ```
+#
+# Usage: `createPublicVarSetter <name> <code>`
+#
+# Example:
+# ```sh
+# createPublicVarSetter MY_VAR 'echo "FOO"';
+# echo $MY_VAR;  # Not defined
+#
+# setPublicVar_MY_VAR;
+# source $BASH_ENV;
+# echo $MY_VAR;  # FOO
+# ```
+function createPublicVarSetter() {
+  echo "setPublicVar_$1() { setPublicVar $1 \"$2\"; }" >> $BASH_ENV;
+}