[package] xz_utils/5.2.4: build_requirements logical error in windows mingw env
### Package and Environment Details
  * Package Name/Version: **xz_utils/5.2.4**
  * Operating System+version: **Windows 10**
  * Compiler+version: **msys2 /mingw64 gcc 8.1**
  * Docker image: **conanio/gcc7**
  * Conan version: **conan 1.21.0**
  * Python version: **Python 3.7.5/none**


### Conan profile
```
Configuration:
[settings]
arch=x86_64
arch_build=x86_64
build_type=Debug
compiler=gcc
compiler.cppstd=14
compiler.exception=seh
compiler.libcxx=libstdc++11
compiler.threads=posix
compiler.version=8.1
os=Windows
os_build=Windows
[options]
[build_requires]
[env]
CONAN_CMAKE_GENERATOR=MinGW Makefiles
```

```
[requires]
xz_utils/5.2.4
```


### Steps to reproduce

default `build_requirements` logical error in windows mingw env.
```
    @property
    def _is_mingw_windows(self):
        # Linux MinGW doesn't require MSYS2 bash obviously
        return self.settings.compiler == 'gcc' and self.settings.os == 'Windows' and os.name == 'nt'

    def build_requirements(self):
        if self._is_mingw_windows:
            self.build_requires("msys2/20161025")

```

In windows env, with msys2 and mingw both, it not need to loading required. It missed judgment `bash` in windows.
Need to change to:
```
    @property
    def _is_mingw_windows(self):
        # Linux MinGW doesn't require MSYS2 bash obviously
        return self.settings.compiler == 'gcc' and self.settings.os == 'Windows' and os.name == 'nt'

    def build_requirements(self):
        if self._is_mingw_windows and "CONAN_BASH_PATH" not in os.environ:
            self.build_requires("msys2/20190524")
```

diff --git a/recipes/xz_utils/all/conanfile.py b/recipes/xz_utils/all/conanfile.py
index f52dd2174..0ada1ece4 100644
--- a/recipes/xz_utils/all/conanfile.py
+++ b/recipes/xz_utils/all/conanfile.py
@@ -25,7 +25,7 @@ class XZUtils(ConanFile):
         return self.settings.compiler == 'gcc' and self.settings.os == 'Windows' and os.name == 'nt'
 
     def build_requirements(self):
-        if self._is_mingw_windows:
+        if self._is_mingw_windows and "CONAN_BASH_PATH" not in os.environ and tools.os_info.detect_windows_subsystem() != 'msys2':
             self.build_requires("msys2/20161025")
 
     def _effective_msbuild_type(self):
['recipes/xz_utils/all/conanfile.py']