[lit] Fix to make "RUN: env PATH=..." work as intended

There was a bug in llvm-lit related to setting PATH using env
in the internal shell.

The new PATH wasn't used when looking up the command to be executed.
So when doing things like this in a test case
  RUN: mkdir %t
  RUN: env PATH=%t program ...
the internal shell would search for "program" using the orignal PATH
and not the PATH set by env when preceeding the command.

It seems like this was a simple mistake in commit 57782ef,
since the logic to pick a PATH from the cmd_shenv instead of shenv
actually was added in that patch, but the resulting path wasn't used.

Author: bjope

llvm/utils/lit/lit/TestRunner.py

@@ -945,7 +945,7 @@ def _executeShCmd(cmd, shenv, results, timeoutHelper):
             path = (
                 cmd_shenv.env["PATH"] if "PATH" in cmd_shenv.env else shenv.env["PATH"]
             )
-            executable = lit.util.which(args[0], shenv.env["PATH"])
+            executable = lit.util.which(args[0], path)
         if not executable:
             raise InternalShellError(j, "%r: command not found" % args[0])