install: Setup VS env if we did that during setup

Otherwise we might not find a ninja that was picked up from the Visual
Studio installation.

```
$ meson setup _build
...
Activating VS 15.9.40
...
Found ninja-1.8.2 at "C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\Common7\IDE\CommonExtensions\Microsoft\CMake\Ninja\ninja.EXE"
$ meson compile -C _build
Activating VS 15.9.40
...
$ meson install -C _build
Can't find ninja, can't rebuild test.
```

Fixes https://github.com/mesonbuild/meson/issues/9774

mesonbuild/minstall.py

@@ -25,6 +25,7 @@ import subprocess
 import sys
 import typing as T
 
+from . import build
 from . import environment
 from .backend.backends import (
     InstallData, InstallDataBase, InstallEmptyDir, InstallSymlinkData,
@@ -32,7 +33,7 @@ from .backend.backends import (
 )
 from .coredata import major_versions_differ, MesonVersionMismatchException
 from .coredata import version as coredata_version
-from .mesonlib import Popen_safe, RealPathAction, is_windows
+from .mesonlib import Popen_safe, RealPathAction, is_windows, setup_vsenv
 from .scripts import depfixer, destdir_join
 from .scripts.meson_exe import run_exe
 try:
@@ -793,6 +794,8 @@ def run(opts: 'ArgumentType') -> int:
     if not os.path.exists(os.path.join(opts.wd, datafilename)):
         sys.exit('Install data not found. Run this command in build directory root.')
     if not opts.no_rebuild:
+        b = build.load(opts.wd)
+        setup_vsenv(b.need_vsenv)
         if not rebuild_all(opts.wd):
             sys.exit(-1)
     os.chdir(opts.wd)