[Debuginfod] Fix debuginfod unit test when $HOME is not writable.

[noajshu]

Summary

The directory returned from cache_directory may not be writable if the unit tests are run with limited permissions (e.g. if $HOME is not writable).
This fixes a unit test of the debuginfod client to set the cache path to the system temporary directory to avoid this issue.

[estewart08]

LGTM

llvm/unittests/Debuginfod/DebuginfodTests.cpp

Expected<std::string> PathOrErr = getCachedOrDownloadArtifact(
       UniqueKey, /*UrlPath=*/"/null", CacheDir,
       /*DebuginfodUrls=*/{}, /*Timeout=*/std::chrono::milliseconds(1));
   EXPECT_THAT_EXPECTED(PathOrErr, HasValue(CachedFilePath));
 }
 
 // Check that the Debuginfod client returns an Error when it fails to find an
 // artifact.
 TEST(DebuginfodClient, CacheMiss) {
+  // Set the cache path to a temp directory to avoid permissions issues if $HOME
+  // is not writable.
+  SmallString<32> TempDir;
+  sys::path::system_temp_directory(true, TempDir);
+  setenv("DEBUGINFOD_CACHE_PATH", TempDir.c_str(),
+         /*replace=*/1);
   // Ensure there are no urls to guarantee a cache miss.
   setenv("DEBUGINFOD_URLS", "", /*replace=*/1);
   HTTPClient::initialize();
   Expected<std::string> PathOrErr = getCachedOrDownloadArtifact(
       /*UniqueKey=*/"nonexistent-key", /*UrlPath=*/"/null");
   EXPECT_THAT_EXPECTED(PathOrErr, Failed<StringError>());
 }