feat: allow setting GST_DEBUG env at runtime

[prlanzarin]

GST_DEBUG cannot be set at run time (i.e.: as an env var for the docker container) since it's always overwritten by the application. This makes debugging the app with more detailed gstreamer logs a bit more convoluted - requires building a modified container, etc.

Allow setting the GST_DEBUG flag at runtime. If set externally, it'll overwrite the application-generated GST_DEBUG entirely. If unset, the current behavior is preserved.

[CLAassistant]

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