[clang] [Clang] Propagate 'SystemDrive' environment variable for unit tests (PR #90478)

[via cfe-commits]

================
@@ -32,6 +32,8 @@
 
 if "HOME" in os.environ:
     config.environment["HOME"] = os.environ["HOME"]
+if "SystemDrive" in os.environ:
+    config.environment["SystemDrive"] = os.environ["SystemDrive"]
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cor3ntin wrote:

can we do something like

```py
# Propagate directories. Wrong values might be used on windows otherwise
for key in ["TMP", "TEMP", "HOME", "SystemDrive"]:
  config.environment[key] = os.environ[key]
```

https://github.com/llvm/llvm-project/pull/90478