cmake: Fix default installation directory backslash on Windows

[qhouz]

Description

Use file(TO_CMAKE_PATH ...) to normalize the path obtained from $ENV{ALLUSERSPROFILE}
before assigning it to CMAKE_INSTALL_PREFIX.

Motivation and Context

$ENV{ALLUSERSPROFILE} on Windows expands to C:\ProgramData — a path with backslashes.
CMake treats backslashes as escape sequences inside strings, causing a parse error when
running cmake --install:

CMake Error at build_x64/cmake_install.cmake:5 (set):
  Syntax error in cmake code at
    .../build_x64/cmake_install.cmake:5
  when parsing string
    C:\ProgramData/obs-studio/plugins
  Invalid character escape '\P'.

How Has This Been Tested?

Tested on Windows 11 x64 with Visual Studio 2026.
Ran cmake --preset windows-x64, cmake --build --preset windows-x64 --config Debug,
and cmake --install build_x64 --config Debug — no errors.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have read the contributing document.
• My code has been run through [clang-format](https://github.com/obsproject/obs-studio/blob/master/.clang-format).
• My code follows the project's [style guidelines](https://github.com/obsproject/obs-studio/blob/master/CODESTYLE.md)
• My code is not on the master branch.
• My code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

Note: clang-format and documentation checks are not applicable —
this change only modifies a CMake script file.