Allow custom LibreOffice binary path via environment variable

[daido1976]

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I would like to update the LibreOffice version in shelfio/libreoffice-lambda-base-image. However, since the LibreOffice binary version and path are hardcoded in the following location, if there is an application already in operation, changes need to be applied simultaneously to both repositories, making it challenging to upgrade the version. https://github.com/shelfio/aws-lambda-libreoffice/blob/6e23c3c3382c2c9829b50982a718fceeacce83e9/src/convert.ts#L17

With this PR, the LO_BINARY_PATH environment variable allows injecting the version and path of LibreOffice, simplifying the upgrade process.