fix: 'build:json' script on windows

[michelmueller]

Please always provide the GitHub issue(s) your PR is for, as well as test URLs where your change can be observed (before and after):

Fix #45

Test URLs: Before: https://main--aem-boilerplate-xwalk--adobe-rnd.hlx.live After: https://build-json-win--aem-boilerplate-xwalk--adobe-rnd.hlx.live

[aem-code-sync[bot]]

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