increase timeout for windows C++ distribtests (#32669)

After switching to grpc-win2019 kokoro image, the windows C++
distribtest got slightly slower (from about ~1500sec to ~1650sec), which
got us too close to the 1800sec time limit and thus we've seen the
windows distribtests occasionally time out.

I'd still like to look into speeding up the distribtests, but for now we
don't want frequest test timeouts in the CI.

example failure: 

https://fusion2.corp.google.com/ci;ids=1923284992/kokoro/prod:grpc%2Fcore%2Fmaster%2Fwindows%2Fgrpc_distribtests_standalone/activity/996fd2cf-bef3-4490-85da-a52765d84127/log

```
2023-03-20 23:43:44,740 TIMEOUT: distribtest.cpp_windows_x86_cmake_as_externalproject [pid=5984, time=1800.1sec]
```

tools/run_tests/artifacts/distribtest_targets.py

@@ -335,7 +335,7 @@ class CppDistribTest(object):
                 self.name,
                 ['test\\distrib\\cpp\\run_distrib_test_%s.bat' % self.testcase],
                 environ={},
-                timeout_seconds=30 * 60,
+                timeout_seconds=45 * 60,
                 use_workspace=True)
         else:
             raise Exception("Not supported yet.")