fix(@angular/build): normalize --include paths to posix

[alan-agius4]

This fixes an issue where relative includes didn't work when using backslashes in windows.

Closes #31829

---

Green run: https://github.com/angular/angular-cli/actions/runs/19467531139/job/55706346256?pr=31854

Running test "tests/vitest/include" (1 of 1)...
==========================================================================================
Running `npm "install" "vitest@^4.0.8" "jsdom@^27.1.0" "--save-dev"` [silent]...
CWD: D:\tmp_dir\angular-cli-e2e-1UgCNT\e2e-test\test-project
==========================================================================================
Running `ng "test" "--include" "D:\tmp_dir\angular-cli-e2e-1UgCNT\e2e-test\test-project\src\app\app.spec.ts"` [silent]...
CWD: D:\tmp_dir\angular-cli-e2e-1UgCNT\e2e-test\test-project
==========================================================================================
Running `ng "test" "--include" "src\app\app.spec.ts"` [silent]...
CWD: D:\tmp_dir\angular-cli-e2e-1UgCNT\e2e-test\test-project
==========================================================================================

[ngbot]

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If you want your PR to be merged, it has to pass all the CI checks.
If you can't get the PR to a green state due to flakes or broken main, please try rebasing to main and/or restarting the CI job. If that fails and you believe that the issue is not due to your change, please contact the caretaker and ask for help.

[ngbot]

I see that you just added the action: merge label, but the following checks are still failing:
     status "mergeability" is failing
If you want your PR to be merged, it has to pass all the CI checks.
If you can't get the PR to a green state due to flakes or broken main, please try rebasing to main and/or restarting the CI job. If that fails and you believe that the issue is not due to your change, please contact the caretaker and ask for help.

[dgp1130]

This PR was merged into the repository. The changes were merged into the following branches:

• main: d5b4d03
• 21.0.x: 59ff867