[llvm] [lit] Fix to make "RUN: env PATH=..." work as intended (PR #165308)

[Alexander Richardson via llvm-commits]

================
@@ -1,7 +1,7 @@
 ## Tests env command for setting the PATH variable.
 
 # The test is using /bin/sh. Limit to system known to have /bin/sh.
-# REQUIRED: system-linux
+# REQUIRES: system-linux
----------------
arichardson wrote:

We have quite a few tests that have this requires even though they should also work on other systems such as freebsd. Should it be the following instead? 
```suggestion
# UNSUPPORTED: system-windows
```

https://github.com/llvm/llvm-project/pull/165308