Fix the unstable behavior of 'test_where_warning' (#4747)

An unexpected 'test_where_warning' failure occurred if the test was
restarted. This happens because the compiler uses the cache and does not
write a warning. The environment parameter 'TRITON_ALWAYS_COMPILE' is
now used to skip the cache.

- [x] I am not making a trivial change, such as fixing a typo in a
comment.

- [x] I have written a PR description following these
  [rules](https://cbea.ms/git-commit/#why-not-how).

- [x] I have run `pre-commit run --from-ref origin/main --to-ref HEAD`.

- Select one of the following.
  - [ ] I have added tests.
    - `/test` for `lit` tests
    - `/unittest` for C++ tests
    - `/python/test` for end-to-end tests
  - [x] This PR does not need a test because `FILL THIS IN`.

- Select one of the following.
  - [x] I have not added any `lit` tests.
- [ ] The `lit` tests I have added follow these [best
practices](https://mlir.llvm.org/getting_started/TestingGuide/#filecheck-best-practices),
including the "tests should be minimal" section. (Usually running Python
code
    and using the instructions it generates is not minimal.)

Signed-off-by: Kirill Suvorov <[email protected]>

python/test/unit/language/test_compile_errors.py

@@ -341,7 +341,7 @@ def kernel(a=1, B: tl.constexpr = ""):
     triton.compile(triton.compiler.ASTSource(fn=kernel, signature={'a': 'i32'}, constants={'B': ""}))
 
 
-def test_where_warning():
+def test_where_warning(fresh_triton_cache):
 
     @triton.jit
     def kernel():