test: remove GITHUB_OUTPUT env var during tests

GITHUB_OUTPUT belongs to the action which executes these tests - we don't want to affect it.

There will be a follow up to set GITHUB_OUTPUT to something temporary, so that we can actually test things end-to-end, under real-life conditions.

(cherry picked from commit 554b17c)

Author: dominykas

.github/actions/prepare-node-test-matrix-action/test/index.js

@@ -41,6 +41,8 @@ exports.getOutput = function (now, pkg) {
 
 exports.main = function () {
 
+    delete process.env.GITHUB_OUTPUT;
+
     const originalEnv = { ...process.env };
 
     // does not fail with defaults, cannot assert, as output is dependent on date