Add note for SPRING_PROFILES_ACTIVE in contributing guide

See spring-projectsgh-24478

CONTRIBUTING.adoc

@@ -67,7 +67,9 @@ added after the original pull request but before a merge.
 * Add some Javadocs.
 * A few unit tests would help a lot as well -- someone has to do it.
 * Verification tasks, including tests and Checkstyle, can be executed
-  by running `./gradlew check` from the project root.
+  by running `./gradlew check` from the project root. Note that `SPRING_PROFILES_ACTIVE`
+  environment variable might affect the result of tests, so in that case, you can prevent
+  it by running `unset SPRING_PROFILES_ACTIVE` before running the task.
 * If no-one else is using your branch, please rebase it against the current master (or
   other target branch in the main project).
 * When writing a commit message please follow https://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html[these conventions],