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

[Durgadoss R via llvm-commits]

================
@@ -345,7 +346,7 @@ def version_int(ver):
 
 
 ptxas_executable = (
-    os.environ.get("LLVM_PTXAS_EXECUTABLE", None) or config.ptxas_executable
+    os.environ.get("LLVM_PTXAS_EXECUTABLE", None) or config.ptxas_executable or shutil.which("ptxas")
----------------
durga4github wrote:

Can we keep the env-var option intact even when configuring through a cmake option?
I find the environment variable option very useful for testing PTX across multiple versions of `ptxas`.

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