/tmp not writable on tvOS device, breaking Path.GetTempPath

[directhex]

Whilst working on #34007, a new issue was revealed where tests touching /tmp (via shared test infra which sets up a temp directory) fail, due to /tmp not being writable.

The existing libraries code uses the TMPDIR env var if it exists, and falls back to /tmp - and helpfully, this is valid on the tvOS simulator, just not on physical devices.

This will be addressed in #57508 as well, since the issues are related (and the proposed fix to this issue depends on the fix to #34007)

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Issue Details

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Whilst working on #34007, a new issue was revealed where tests touching /tmp (via shared test infra which sets up a temp directory) fail, due to /tmp not being writable.

The existing libraries code uses the TMPDIR env var if it exists, and falls back to /tmp - and helpfully, this is valid on the tvOS simulator, just not on physical devices.

This will be addressed in #57508 as well, since the issues are related (and the proposed fix to this issue depends on the fix to #34007)

Author:	directhex
Assignees:	-
Labels:	area-System.IO, blocking-release, in pr
Milestone:	-