Make msys live inside overriden config path by default

src/alire/alire-platforms-folders.ads

@@ -2,6 +2,10 @@ package Alire.Platforms.Folders is
 
    --  This spec must be fulfilled by bodies for each different OS we support
 
+   --  IMPORTANT NOTE: these are not intended to be used directly in most
+   --  cases; instead, use the ones in Alire.Config.Edit that take into
+   --  account user overriding of the config location.
+
    function Config return Absolute_Path;
    --  Folder where alire will store its global configuration, indexes, and
    --  any other global data. Deleting it is akin to running alr afresh for

src/alire/os_windows/alire-config-builtins-windows.ads

@@ -1,4 +1,4 @@
-with Alire.Platforms.Folders;
+with Alire.Config.Edit;
 
 package Alire.Config.Builtins.Windows is
 
@@ -19,7 +19,7 @@ package Alire.Config.Builtins.Windows is
    Msys2_Install_Dir : constant Builtin := New_Builtin
      (Key  => "msys2.install_dir",
       Kind => Cfg_Absolute_Path,
-      Def  => Platforms.Folders.Cache / "msys64",
+      Def  => Config.Edit.Cache_Path / "msys64",
       Help =>
         "Directory where Alire will detect and/or install" &
         " msys2 system package manager. (Windows only)");

testsuite/drivers/alr.py

@@ -47,6 +47,19 @@ def prepare_env(config_dir, env):
     #  We pass config location explicitly in the following calls since env is
     #  not yet applied (it's just a dict to be passed later to subprocess)
 
+    # Set the msys2 path to the default one, so that every test does not try to
+    # reinstall a new copy in their private config
+    if platform.system() == "Windows":
+        run_alr("-c", config_dir, "config", "--global",
+                "--set", "msys2.install_dir",
+                os.path.join(os.environ.get("LocalAppData"), "alire", "msys64"))
+
+        # And, to make completely sure, disable msys2 installation, as tests
+        # use their own config that should not rely on a fresh msys2 anyway. On
+        # GitHub CI, msys2 is set up in advance.
+        run_alr("-c", config_dir, "config", "--global",
+                "--set", "msys2.do_not_install", "true")
+
     # Disable autoconfig of the community index, to prevent unintended use of
     # it in tests, besides the overload of fetching it
     run_alr("-c", config_dir, "config", "--global",