[skip ci] Skip suspicious fpm tests on GH actions & macOS

There have been misterious macOS failures with the following error for a long
time, which would usually happen 1-2x per nightly run:

> The hosted runner lost communication with the server. Anything in your
> workflow that terminates the runner process, starves it for CPU/Memory, or
> blocks its network access can cause this error.

After way too much debugging, it looks like this is caused by the two fpm tests
skipped in this commit. When there's a failure, the responsible test will run
for at least 120 seconds until the job is eventually killed by GitHub Actions.
It's not clear yet why the tests stall.

Debugging this is a PITA because the GitHub Actions log gets partially lost for
killed jobs. It took an external log server to actually figure out where the job
fails. Let's disable the tests first to be sure this actually solves the issue.

sapi/fpm/tests/bug74083-concurrent-reload.phpt

@@ -4,6 +4,7 @@ Concurrent reload signals should not kill PHP-FPM master process. (Bug: #74083)
 <?php
 include "skipif.inc";
 if (getenv("SKIP_SLOW_TESTS")) die("skip slow test");
+if (getenv("GITHUB_ACTIONS") && PHP_OS_FAMILY === "Darwin") die("flaky Potential cause for GitHub Actions crash");
 ?>
 --FILE--
 <?php

sapi/fpm/tests/bug76601-reload-child-signals.phpt

@@ -4,6 +4,7 @@ FPM: bug76601 children should not ignore signals during concurrent reloads
 <?php
 include "skipif.inc";
 if (getenv("SKIP_SLOW_TESTS")) die("skip slow test");
+if (getenv("GITHUB_ACTIONS") && PHP_OS_FAMILY === "Darwin") die("flaky Potential cause for GitHub Actions crash");
 ?>
 --FILE--
 <?php