Bug 1996563 - Generate perfherder_data artifacts for jobs running fetch-content

[junngo]

Some workers run in dynamically generated working directories (e.g., D:\task_17612\ or /home/cltbld/tasks/task_17612/), which means the upload path isn’t always static.
Previously, we relied on UPLOAD_DIR when generating the artifact, but not all jobs expose this environment variable, which caused inconsistencies.
To fix this, we added a new environment variable, PERFHERDER_FETCH_CONTENT_JSON_PATH[0]. It gives us a clear and reliable file path, so we can always generate the perfherder-data-fetch-content.json artifact correctly on any worker.

[0]
Related patch (mozilla-central): https://phabricator.services.mozilla.com/D270831
Bugzilla link: https://bugzilla.mozilla.org/show_bug.cgi?id=1996563

[junngo]

I modified the code directly in mozilla-central to test it. Here is the CI test I ran. [0]
We can see that the perfherder-data-fetch-content.json artifact is generated correctly.
If there is anything else I should change, please feel free to let me know :)

[0]
https://treeherder.mozilla.org/jobs?repo=try&revision=70d74fb69a4c26602893c0ff7d9bdaa5f59ef18b
https://treeherder.mozilla.org/jobs?repo=try&revision=70d74fb69a4c26602893c0ff7d9bdaa5f59ef18b&selectedTaskRun=WJyjic9zQOqFbM6ctfMdyA.0