Conditionally reenable hard_link tests on android

[the8472]

#102757 disabled tests for hardlinks on android. This is unfortunate because it decreases our test-coverage, I ran into this in #159346.
Android is tier2, but we do in fact run std tests on the arm-android CI jobs.

It's not clear from the discussion on the original PR whether this was motivated by our CI or external users?
It'd be great if we could make these tests opt-{in,out} and then enable them on our CI and if necessary adjust the emulator to run with the necessary permissions to actually run the tests.

CC @pcc

[RalfJung]

Cc @chriswailes @jfgoog @maurer @pirama-arumuga-nainar

[maurer]

I believe (can't check now, on vacation, not touching work resources) that this was changed for running libstd on new devices, not on your CI - your CI is using. With the work going on over at rust-lang/libc to try switching to cuttlefish (which would allow us to switch to more modern Android for the test environment), we'd run into the same selinux rules, but you could disable that via setenforce 0.

tl;dr: I think it was done for running std tests on a generic android device. If you have root on it (e.g. an emulator) and the test fails for permission reasons, you can prepare by running setenforce 0, and it will ignore the SELinux permission checks.

[the8472]

Ok, hopefully we can do the if env("CI") thing we do in a few other places to make it mandatory in CI and on devboxes we do runtime checks that will ignore when they lack root.