system/icu: fails tests because of tzdata update

During GCC 13 rebuilds, I noticed icu failed its test suite. It didn't seem related to GCC, and indeed, it isn't:

         TestGenericAPI {
         FAIL: detectHostTimeZone()'s raw offset != host timezone's offset
      
         } ERRORS (1) in TestGenericAPI
Errors in total: 1.
            TestGenericAPI
         TimeZoneTest
      format
   
--------------------------------------
Elapsed Time: 00:02:03.766
make: *** [Makefile:111: check-local] Error 1

Adding more detail to the error log gave:

         TestGenericAPI {
         FAIL: detectHostTimeZone()'s raw offset -21600000 != host timezone's offset -25200000
      
         } ERRORS (1) in TestGenericAPI

There have been numerous rewrites of the timezone data used in ICU since the release we are shipping. I don't want to update ICU at the same time as GCC, so temporarily I will use TZ=UTC which does make the tests pass. We can remove that workaround when we update ICU.

assigned to @awilfox

changed title from sys-libs/icu: fails tests because of tzdata update to system/icu: fails tests because of tzdata update

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