[llvm] [llvm-lit][NVPTX] Use the shell's `ptxas` if `LLVM_PTXAS_EXECUTABLE` isn't set (PR #132810)

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``````````bash
darker --check --diff -r 1f27ff91b3104f3d2038324f09fb9cab2c75d037...ad13f699d110761e3ad861eb7b9e43c7e0c9b3c6 llvm/test/lit.cfg.py
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View the diff from darker here.
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``````````diff
--- lit.cfg.py	2025-03-21 21:37:12.000000 +0000
+++ lit.cfg.py	2025-03-24 18:55:26.654802 +0000
@@ -344,11 +344,13 @@
         ]
     )
 
 
 ptxas_executable = (
-    os.environ.get("LLVM_PTXAS_EXECUTABLE", None) or config.ptxas_executable or shutil.which("ptxas")
+    os.environ.get("LLVM_PTXAS_EXECUTABLE", None)
+    or config.ptxas_executable
+    or shutil.which("ptxas")
 )
 if ptxas_executable:
     enable_ptxas(ptxas_executable)
 
 llvm_config.add_tool_substitutions(tools, config.llvm_tools_dir)

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https://github.com/llvm/llvm-project/pull/132810