Use temp directory as TestExecutionDirectory in RunTestsOnHelix.sh

[akoeplinger]

I looked into using the temp directory on Unix like we do on Windows and it uncovered two issues (#3923 was an earlier report)

1. Helix sets $TMPDIR to /tmp/helix but on macOS /tmp is a symlink to /private/tmp and a bunch of tests then fail since parts of the sdk see /tmp and other parts see the resolved /private/tmp path and checks for path equality and other similar logic fails (see also .NET SDK fails to resolve transitive references in standard macOS $TMPDIR and other symlinked directories #37687)
   -> this was relatively easy to solve by making sure we use the resolved path as the TestExecutionDirectory. Ideally we'd solve the underlying symlink equality issue but that's a bigger change.

2. the test GivenWorkloadUpdateAcrossFeatureBandsItUpdatesPacks started failing in CI but only on Linux. I tried reproducing it locally but it always passed. Then I stepped through it under the debugger and noticed that I got this exception here:

   System.IO.IOException : The process cannot access the file '/Users/alexander/dev/sdk/artifacts/tmp/Debug/GivenWorkload---B940FC6C/dotnet/metadata/workloads/InstalledPacks/v1/mock-pack-2/2.0.0/5.0.100' because it is being used by another process.

   ... but we're swallowing the exception so the test accidentally passed because the workload GC was aborted.

   I tracked it down to us not closing the file stream created by File.Create() during the test, once I fixed that the test now consistently failed everywhere (it asserts that the file should exist but it gets deleted during workload GC). This was fixed by mocking up a 5.0.1xx SDK install so that the installation records for that feature band won't be deleted.

We're now correctly closing all file streams created by File.Create() in the codebase.

[dsplaisted]

@akoeplinger @joeloff I pushed a commit which should fix the workload test failure.

[akoeplinger]

@dsplaisted thanks, I updated the PR description to capture the root cause.