msys2: clear package cache (#27469)

it removes the content of bin\msys64\var\cache\pacman\pkg
saving more than 100MB

Author: ericLemanissier

recipes/msys2/all/conanfile.py

@@ -144,6 +144,7 @@ def _do_build(self):
             for package in ['pkgconf']:
                 if self.run(f'bash -l -c "pacman -Qq {package}"', ignore_errors=True, quiet=True) == 0:
                     self.run(f'bash -l -c "pacman -Rs -d -d {package} --noconfirm"')
+            self.run(f'bash -l -c "pacman -Scc --noconfirm"')
 
         self._kill_pacman()