Convert paths in venv activate script when using MinGW

With python#112508 the check to change paths when running on Windows was changed from using the non-posix environment variable `$OSTYPE` to using `uname` instead.

However this missed the fact that when running under MinGW shell, uname reports `MINGW*` (`$OSTYPE` is still `msys`).

This results in `$PATH` being set to something like `D:\a\github-actions-shells\github-actions-shells\venv/Scripts:…`, instead of `/d/a/github-actions-shells/github-actions-shells/venv/Scripts`.

Notably, the Git Bash shell for Windows behaves like this, and this is also the bash shell that’s used for GitHub Actions Windows runners.

Author: julienp

Lib/venv/scripts/common/activate

@@ -38,8 +38,8 @@ deactivate nondestructive
 
 # on Windows, a path can contain colons and backslashes and has to be converted:
 case "$(uname)" in
-    CYGWIN*|MSYS*)
-        # transform D:\path\to\venv to /d/path/to/venv on MSYS
+    CYGWIN*|MSYS*|MINGW*)
+        # transform D:\path\to\venv to /d/path/to/venv on MSYS and MINGW
         # and to /cygdrive/d/path/to/venv on Cygwin
         VIRTUAL_ENV=$(cygpath "__VENV_DIR__")
         export VIRTUAL_ENV