add_install_script() / ninja install fail with "[script.sh]: No such file or directory" / "FAILED: install script 'bash [script.sh]'"

Describe the bug
ninja install does not work on MSYS2/MinGW64. It 'cannot find' even the simplest install script.

$ ninja install
[0/1] Installing files.
Running custom install script 'bash C:/msys64/home/user.name/src/tmp/test.sh HELLO'
--- stdout ---

--- stderr ---
/bin/bash: C:/msys64/home/user.name/src/tmp/test.sh: No such file or directory

FAILED: install script 'bash C:/msys64/home/user.name/src/tmp/test.sh HELLO' exit code 127, stopped
FAILED: meson-install
"C:/msys64/mingw64/bin/meson" "install" "--no-rebuild"
ninja: build stopped: subcommand failed.

To Reproduce
meson.build:

project('test')
meson.add_install_script('test.sh')

tesh.sh:

#!/bin/bash
echo hiya

Expected behavior
The script should be trivially found and run!

system parameters

• Is this a cross build or just a plain native build (for the same computer)? native
• what operating system Windows 10 with msys2-runtime 3.1.7-6
• what Python version are you using 3.8.8
• what meson --version 0.57.1
• what ninja --version if it's a Ninja build 1.10.2

Start with the meson.build example and the meson.add_install_script('test.sh') entry point, then reproduce the failure with ninja install on Windows 10 under MSYS2/MinGW64. Trace how the referenced test.sh path is passed to the install script and verify that the script is found and runs successfully.