[clang] Disable Driver/env.c on AIX (NFC) (PR #161874)

[Fangrui Song via cfe-commits]

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@@ -1,5 +1,7 @@
 // Some assertions in this test use Linux style (/) file paths.
-// UNSUPPORTED: system-windows
+// UNSUPPORTED: system-windows, system-aix
+// FIXME: Use LIBPATH on AIX
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MaskRay wrote:

We should remove the FIXME. It's at best a TODO, but I think an AIX test, if we ever add, will reside in a different file.

https://github.com/llvm/llvm-project/pull/161874